title: WebView Error on Navigation View with Fragment
body: <p>I am making a simple application but getting error, here is my
  MainActivity.java</p>
  
  <pre><code>package com.example.intawebapp.activity;
  
  import android.content.Intent;
  import android.os.Bundle;
  import android.os.Handler;
  import android.support.design.widget.FloatingActionButton;
  import android.support.design.widget.NavigationView;
  import android.support.design.widget.Snackbar;
  import android.support.v4.app.Fragment;
  import android.support.v4.app.FragmentTransaction;
  import android.support.v4.view.GravityCompat;
  import android.support.v4.widget.DrawerLayout;
  import android.support.v7.app.ActionBarDrawerToggle;
  import android.support.v7.app.AppCompatActivity;
  import android.support.v7.widget.Toolbar;
  import android.view.Menu;
  import android.view.MenuItem;
  import android.view.View;
  import android.webkit.WebSettings;
  import android.webkit.WebView;
  import android.webkit.WebViewClient;
  import android.widget.ImageView;
  import android.widget.TextView;
  import android.widget.Toast;
  
  import com.bumptech.glide.Glide;
  import com.bumptech.glide.load.engine.DiskCacheStrategy;
  
  import com.example.intawebapp.R;
  import com.example.intawebapp.fragment.HomeFragment;
  import com.example.intawebapp.fragment.NotificationsFragment;
  import com.example.intawebapp.fragment.SettingsFragment;
  import com.example.intawebapp.other.CircleTransform;
  import com.example.intawebapp.fragment.PortalFragment;
  import com.example.intawebapp.fragment.ESSFragment;
  
  public class MainActivity extends AppCompatActivity {
  
  
  
      WebView myWebView;
      private NavigationView navigationView;
      private DrawerLayout drawer;
      private View navHeader;
      private ImageView imgNavHeaderBg, imgProfile;
      private TextView txtName, txtWebsite;
      private Toolbar toolbar;
      private FloatingActionButton fab;
  
      // urls to load navigation header background image
      // and profile image
      private static final String urlNavHeaderBg = "http://example.com/nav-menu-header-bg.jpg";
      private static final String urlProfileImg = "http://example.com/prof";
  
      // index to identify current nav menu item
      public static int navItemIndex = 0;
  
      // tags used to attach the fragments
      private static final String TAG_HOME = "home";
  
      private static final String TAG_PORTAL = "portal";
      private static final String TAG_ESS = "ess";
  
      private static final String TAG_NOTIFICATIONS = "notifications";
      private static final String TAG_SETTINGS = "settings";
      public static String CURRENT_TAG = TAG_HOME;
  
      // toolbar titles respected to selected nav menu item
      private String[] activityTitles;
  
      // flag to load home fragment when user presses back key
      private boolean shouldLoadHomeFragOnBackPress = true;
      private Handler mHandler;
  
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
          toolbar = (Toolbar) findViewById(R.id.toolbar);
          setSupportActionBar(toolbar);
  
          mHandler = new Handler();
  
          drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
          navigationView = (NavigationView) findViewById(R.id.nav_view);
          fab = (FloatingActionButton) findViewById(R.id.fab);
  
          // Navigation view header
          navHeader = navigationView.getHeaderView(0);
          txtName = (TextView) navHeader.findViewById(R.id.name);
          txtWebsite = (TextView) navHeader.findViewById(R.id.website);
          imgNavHeaderBg = (ImageView) navHeader.findViewById(R.id.img_header_bg);
          imgProfile = (ImageView) navHeader.findViewById(R.id.img_profile);
  
          myWebView = (WebView) findViewById(R.id.myWebView);
  
          WebSettings webSettings = myWebView.getSettings();
          webSettings.setJavaScriptEnabled(true);
          myWebView.setWebViewClient(new MyWebViewClient());
  
  
          // load toolbar titles from string resources
          activityTitles = getResources().getStringArray(R.array.nav_item_activity_titles);
  
          fab.setOnClickListener(new View.OnClickListener() {
              @Override
              public void onClick(View view) {
                  Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                          .setAction("Action", null).show();
              }
          });
  
          // load nav menu header data
          loadNavHeader();
  
          // initializing navigation menu
          setUpNavigationView();
  
          if (savedInstanceState == null) {
              navItemIndex = 0;
              CURRENT_TAG = TAG_HOME;
              loadHomeFragment();
          }
      }
  
      /***
       * Load navigation menu header information
       * like background image, profile image
       * name, website, notifications action view (dot)
       */
      private void loadNavHeader() {
          // name, website
          txtName.setText("My Name");
          txtWebsite.setText("www.example.com");
  
          // loading header background image
          Glide.with(this).load(urlNavHeaderBg)
                  .crossFade()
                  .diskCacheStrategy(DiskCacheStrategy.ALL)
                  .into(imgNavHeaderBg);
  
          // Loading profile image
          Glide.with(this).load(urlProfileImg)
                  .crossFade()
                  .thumbnail(0.5f)
                  .bitmapTransform(new CircleTransform(this))
                  .diskCacheStrategy(DiskCacheStrategy.ALL)
                  .into(imgProfile);
  
          // showing dot next to notifications label
          navigationView.getMenu().getItem(3).setActionView(R.layout.menu_dot);
      }
  
      /***
       * Returns respected fragment that user
       * selected from navigation menu
       */
      private void loadHomeFragment() {
          // selecting appropriate nav menu item
          selectNavMenu();
  
          // set toolbar title
          setToolbarTitle();
  
          // if user select the current navigation menu again, don't do anything
          // just close the navigation drawer
          if (getSupportFragmentManager().findFragmentByTag(CURRENT_TAG) != null) {
              drawer.closeDrawers();
  
              // show or hide the fab button
              toggleFab();
              return;
          }
  
          // Sometimes, when fragment has huge data, screen seems hanging
          // when switching between navigation menus
          // So using runnable, the fragment is loaded with cross fade effect
          // This effect can be seen in GMail app
          Runnable mPendingRunnable = new Runnable() {
              @Override
              public void run() {
                  // update the main content by replacing fragments
                  Fragment fragment = getHomeFragment();
                  FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
                  fragmentTransaction.setCustomAnimations(android.R.anim.fade_in,
                          android.R.anim.fade_out);
                  fragmentTransaction.replace(R.id.frame, fragment, CURRENT_TAG);
                  fragmentTransaction.commitAllowingStateLoss();
              }
          };
  
          // If mPendingRunnable is not null, then add to the message queue
          if (mPendingRunnable != null) {
              mHandler.post(mPendingRunnable);
          }
  
          // show or hide the fab button
          toggleFab();
  
          //Closing drawer on item click
          drawer.closeDrawers();
  
          // refresh toolbar menu
          invalidateOptionsMenu();
      }
  
      private Fragment getHomeFragment() {
          switch (navItemIndex) {
              case 0:
                  // home
                  HomeFragment homeFragment = new HomeFragment();
                  return homeFragment;
              case 1:
                  // portal
                  PortalFragment portalFragment = new PortalFragment();
                  return portalFragment;
              case 2:
                  // ESS fragment
                  ESSFragment essFragment = new ESSFragment();
                  return essFragment;
              case 3:
                  // notifications fragment
                  NotificationsFragment notificationsFragment = new NotificationsFragment();
                  return notificationsFragment;
  
              case 4:
                  // settings fragment
                  SettingsFragment settingsFragment = new SettingsFragment();
                  return settingsFragment;
              default:
                  return new HomeFragment();
          }
      }
  
      private void setToolbarTitle() {
          getSupportActionBar().setTitle(activityTitles[navItemIndex]);
      }
  
      private void selectNavMenu() {
          navigationView.getMenu().getItem(navItemIndex).setChecked(true);
      }
  
      private class MyWebViewClient extends WebViewClient {
          @Override
          public boolean shouldOverrideUrlLoading(WebView view, String url) {
              myWebView.loadUrl(url);
              return true;
          }
      }
  
      private void setUpNavigationView() {
          //Setting Navigation View Item Selected Listener to handle the item click of the navigation menu
          navigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
  
              // This method will trigger on item Click of navigation menu
              @Override
              public boolean onNavigationItemSelected(MenuItem menuItem) {
  
                  //Check to see which item was being clicked and perform appropriate action
                  switch (menuItem.getItemId()) {
                      //Replacing the main content with ContentFragment Which is our Inbox View;
                      case R.id.home:
                          navItemIndex = 0;
                          CURRENT_TAG = TAG_HOME;
                          break;
                      case R.id.nav_portal:
                          navItemIndex = 1;
                          CURRENT_TAG = TAG_PORTAL;
                          break;
                      case R.id.nav_ess:
                          navItemIndex = 2;
                          CURRENT_TAG = TAG_ESS;
                          break;
                      case R.id.nav_notifications:
                          navItemIndex = 3;
                          CURRENT_TAG = TAG_NOTIFICATIONS;
                          break;
                      case R.id.nav_settings:
                          navItemIndex = 4;
                          CURRENT_TAG = TAG_SETTINGS;
                          break;
                      case R.id.nav_about_us:
                          // launch new intent instead of loading fragment
                          startActivity(new Intent(MainActivity.this, AboutUsActivity.class));
                          drawer.closeDrawers();
                          return true;
                      case R.id.nav_exit:
                          // launch new intent instead of loading fragment
                          startActivity(new Intent(MainActivity.this, ExitActivity.class));
                          drawer.closeDrawers();
                          return true;
                      default:
                          navItemIndex = 0;
                  }
  
                  //Checking if the item is in checked state or not, if not make it in checked state
                  if (menuItem.isChecked()) {
                      menuItem.setChecked(false);
                  } else {
                      menuItem.setChecked(true);
                  }
                  menuItem.setChecked(true);
  
                  loadHomeFragment();
  
                  return true;
              }
          });
  
  
          ActionBarDrawerToggle actionBarDrawerToggle = new ActionBarDrawerToggle(this, drawer, toolbar, R.string.openDrawer, R.string.closeDrawer) {
  
              @Override
              public void onDrawerClosed(View drawerView) {
                  // Code here will be triggered once the drawer closes as we dont want anything to happen so we leave this blank
                  super.onDrawerClosed(drawerView);
              }
  
              @Override
              public void onDrawerOpened(View drawerView) {
                  // Code here will be triggered once the drawer open as we dont want anything to happen so we leave this blank
                  super.onDrawerOpened(drawerView);
              }
          };
  
          //Setting the actionbarToggle to drawer layout
          drawer.setDrawerListener(actionBarDrawerToggle);
  
          //calling sync state is necessary or else your hamburger icon wont show up
          actionBarDrawerToggle.syncState();
      }
  
      @Override
      public void onBackPressed() {
          if (drawer.isDrawerOpen(GravityCompat.START)) {
              drawer.closeDrawers();
              return;
          }
  
          // This code loads home fragment when back key is pressed
          // when user is in other fragment than home
          if (shouldLoadHomeFragOnBackPress) {
              // checking if user is on other navigation menu
              // rather than home
              if (navItemIndex != 0) {
                  navItemIndex = 0;
                  CURRENT_TAG = TAG_HOME;
                  loadHomeFragment();
                  return;
              }
          }
  
          super.onBackPressed();
      }
  
      @Override
      public boolean onCreateOptionsMenu(Menu menu) {
          // Inflate the menu; this adds items to the action bar if it is present.
  
          // show menu only when home fragment is selected
          if (navItemIndex == 0) {
              getMenuInflater().inflate(R.menu.main, menu);
          }
  
          // when fragment is notifications, load the menu created for notifications
          if (navItemIndex == 3) {
              getMenuInflater().inflate(R.menu.notifications, menu);
          }
          return true;
      }
  
      @Override
      public boolean onOptionsItemSelected(MenuItem item) {
          // Handle action bar item clicks here. The action bar will
          // automatically handle clicks on the Home/Up button, so long
          // as you specify a parent activity in AndroidManifest.xml.
          int id = item.getItemId();
  
          //noinspection SimplifiableIfStatement
          if (id == R.id.action_logout) {
              Toast.makeText(getApplicationContext(), "Logout user!", Toast.LENGTH_LONG).show();
              return true;
          }
  
          // user is in notifications fragment
          // and selected 'Mark all as Read'
          if (id == R.id.action_mark_all_read) {
              Toast.makeText(getApplicationContext(), "All notifications marked as read!", Toast.LENGTH_LONG).show();
          }
  
          // user is in notifications fragment
          // and selected 'Clear All'
          if (id == R.id.action_clear_notifications) {
              Toast.makeText(getApplicationContext(), "Clear all notifications!", Toast.LENGTH_LONG).show();
          }
  
          return super.onOptionsItemSelected(item);
      }
  
      // show or hide the fab
      private void toggleFab() {
          if (navItemIndex == 0)
              fab.show();
          else
              fab.hide();
      }
  }
  </code></pre>
  
  <p>and this is my AboutUsActivity</p>
  
  <pre><code>package com.example.intawebapp.activity;
  
  import android.content.Context;
  import android.net.Uri;
  import android.os.Bundle;
  import android.support.v4.app.Fragment;
  import android.view.LayoutInflater;
  import android.view.View;
  import android.view.ViewGroup;
  
  import com.example.intawebapp.R;
  
  public class AboutUsActivity extends Fragment {
  
      private static final String ARG_PARAM1 = "param1";
      private static final String ARG_PARAM2 = "param2";
  
  
      private String mParam1;
      private String mParam2;
  
      private OnFragmentInteractionListener mListener;
  
      public AboutUsActivity() {
          // Required empty public constructor
      }
  
  
      public static AboutUsActivity newInstance(String param1, String param2) {
          AboutUsActivity fragment = new AboutUsActivity();
          Bundle args = new Bundle();
          args.putString(ARG_PARAM1, param1);
          args.putString(ARG_PARAM2, param2);
          fragment.setArguments(args);
          return fragment;
      }
  
      @Override
      public void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          if (getArguments() != null) {
              mParam1 = getArguments().getString(ARG_PARAM1);
              mParam2 = getArguments().getString(ARG_PARAM2);
          }
      }
  
      @Override
      public View onCreateView(LayoutInflater inflater, ViewGroup container,
                               Bundle savedInstanceState) {
          // Inflate the layout for this fragment
          return inflater.inflate(R.layout.activity_about_us, container, false);
      }
  
  
      public void onButtonPressed(Uri uri) {
          if (mListener != null) {
              mListener.onFragmentInteraction(uri);
          }
      }
  
      @Override
      public void onAttach(Context context) {
          super.onAttach(context);
  //        if (context instanceof OnFragmentInteractionListener) {
  //            mListener = (OnFragmentInteractionListener) context;
  //        } else {
  //            throw new RuntimeException(context.toString()
  //                    + " must implement OnFragmentInteractionListener");
  //        }
      }
  
      @Override
      public void onDetach() {
          super.onDetach();
          mListener = null;
      }
  
      public interface OnFragmentInteractionListener {
  
          void onFragmentInteraction(Uri uri);
      }
  }
  </code></pre>
  
  <p>I am getting this error when I choose About from the menu</p>
  
  <pre><code>E/AndroidRuntime: FATAL EXCEPTION: main
                    Process: com.example.intawebapp, PID: 8170
                    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.intawebapp/com.example.intawebapp.activity.AboutUsActivity}: java.lang.ClassCastException: com.example.intawebapp.activity.AboutUsActivity cannot be cast to android.app.Activity
                        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2548)
                        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
                        at android.app.ActivityThread.-wrap12(ActivityThread.java)
                        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
                        at android.os.Handler.dispatchMessage(Handler.java:102)
                        at android.os.Looper.loop(Looper.java:154)
                        at android.app.ActivityThread.main(ActivityThread.java:6077)
                        at java.lang.reflect.Method.invoke(Native Method)
                        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
                        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
                     Caused by: java.lang.ClassCastException: com.example.intawebapp.activity.AboutUsActivity cannot be cast to android.app.Activity
                        at android.app.Instrumentation.newActivity(Instrumentation.java:1078)
                        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2538)
                        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) 
                        at android.app.ActivityThread.-wrap12(ActivityThread.java) 
                        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) 
                        at android.os.Handler.dispatchMessage(Handler.java:102) 
                        at android.os.Looper.loop(Looper.java:154) 
                        at android.app.ActivityThread.main(ActivityThread.java:6077) 
                        at java.lang.reflect.Method.invoke(Native Method) 
                        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865) 
                        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755) 
  Application terminated.
  </code></pre>
  
  <p>I need help to solve this problem. Thanks</p>