title: getFragmentManager returning null pointer exception
body: <p>My getFragmentManager in the function setUpMapIfNeeded() returning <code>null pointer exception</code>. I put my fragment separately from activity_main.xml, here my code :</p>
  
  <p>activity_main.xml :</p>
  
  <pre><code>&lt;FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:tools="http://schemas.android.com/tools"
  android:id="@+id/container"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  tools:context="com.yai.testmap.MainActivity"
  tools:ignore="MergeRootFrame" /&gt; 
  </code></pre>
  
  <h1><b>Updated</b></h1>
  
  <p>fragment_main.xml :</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8" ?&gt;
  
  &lt;RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
   android:layout_width="match_parent"
   android:layout_height="match_parent"&gt;
  
  &lt;!--  MapView--&gt;
  
  &lt;com.google.android.gms.maps.MapView
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:id="@+id/map"
   /&gt;
  </code></pre>
  
  <p>and here my *.java file :</p>
  
  <h1><b>Updated</b></h1>
  
  <pre>
  package com.yai.testmap;
  
  import android.support.v7.app.ActionBarActivity;
  import android.support.v4.app.Fragment;
  import android.os.Bundle;
  import android.view.LayoutInflater;
  import android.view.Menu;
  import android.view.MenuItem;
  import android.view.View;
  import android.view.ViewGroup;
  
  /* FOR ERROR REPPORT */
  import android.util.Log;
  
  /* IMPORT FOR DROPDOWNLIST USED THIS PROJECT */
  import android.widget.CheckBox;
  import android.widget.Spinner;
  import android.widget.Toast;
  import android.widget.AdapterView;
  import android.widget.ArrayAdapter;
  import android.widget.AdapterView.OnItemSelectedListener;
  
  import com.google.android.gms.common.ConnectionResult;
  import com.google.android.gms.common.GooglePlayServicesNotAvailableException;
  import com.google.android.gms.common.GooglePlayServicesUtil;
  /* IMPORT FOR GOOGLE MAP */
  import com.google.android.gms.maps.GoogleMap;
  import com.google.android.gms.maps.MapFragment;
  import com.google.android.gms.maps.MapsInitializer;
  import com.google.android.gms.maps.SupportMapFragment;
  import com.google.android.gms.maps.MapView;
  
  /* FOR GOOGLE MAP TYPE */
  import static com.google.android.gms.maps.GoogleMap.MAP_TYPE_HYBRID;
  import static com.google.android.gms.maps.GoogleMap.MAP_TYPE_NONE;
  import static com.google.android.gms.maps.GoogleMap.MAP_TYPE_NORMAL;
  import static com.google.android.gms.maps.GoogleMap.MAP_TYPE_SATELLITE;
  import static com.google.android.gms.maps.GoogleMap.MAP_TYPE_TERRAIN;
  
  public class MainActivity extends ActionBarActivity implements OnItemSelectedListener {
      private static MapView mMapView;
      private static MapFragment mMapFragment;
      private static GoogleMap mGoogleMap;
      private static CheckBox mTrafficCheckbox, mLocationCheckbox, mBuildingCheckbox, mIndoorCheckbox;
  
      public static class PlaceholderFragment extends Fragment {
  
          public PlaceholderFragment() {
          }
  
          @Override
          public View onCreateView(LayoutInflater inflater, ViewGroup container,
                  Bundle savedInstanceState) {
              View rootView = inflater.inflate(R.layout.fragment_main, container,
                      false);
  
              Spinner spinner = (Spinner) rootView.findViewById(R.id.spinnerOption);
              ArrayAdapter adapter = ArrayAdapter.createFromResource(getActivity(), R.array.spinner_option_string, android.R.layout.simple_spinner_item);
              adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
              spinner.setAdapter(adapter);
              spinner.setOnItemSelectedListener((OnItemSelectedListener) getActivity());
  
              mTrafficCheckbox = (CheckBox) rootView.findViewById(R.id.checkbox_traffic);
              mLocationCheckbox = (CheckBox) rootView.findViewById(R.id.checkbox_location);
              mBuildingCheckbox = (CheckBox) rootView.findViewById(R.id.checkbox_building);
              mIndoorCheckbox = (CheckBox) rootView.findViewById(R.id.checkbox_indoor);
  
  
              MapsInitializer.initialize(getActivity());
  
              switch (GooglePlayServicesUtil.isGooglePlayServicesAvailable(getActivity()) )
              {
              case ConnectionResult.SUCCESS :
                  mMapView = (MapView) rootView.findViewById(R.id.map);   
                  mMapView.onCreate(savedInstanceState);
  
                  if(mMapView != null){
                      mGoogleMap = mMapView.getMap();
                      mGoogleMap.getUiSettings().setMyLocationButtonEnabled(false);
                      mGoogleMap.setMyLocationEnabled(true);
                  }
              }   
  
              return rootView;
          }
      }
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {    
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
  
          if (savedInstanceState == null) {
              getSupportFragmentManager().beginTransaction()
                      .add(R.id.container, new PlaceholderFragment()).commit();
          }       
  
          setUpMapIfNeeded();         
      }   
  
      private void setUpMapIfNeeded(){
          if(mGoogleMap == null){     
              //mMapFragment = (MapFragment) getFragmentManager().findFragmentById(R.id.map);
              mGoogleMap = mMapView.getMap();
          }
      }
  
      private boolean checkReady(){
          if(mGoogleMap == null){
              Toast.makeText(this, R.string.map_not_ready, Toast.LENGTH_SHORT).show();
              return false;
          }
          return true;
      }
  
      private void updateTraffic(){
          if(!checkReady()){
              return;
          }
          mGoogleMap.setTrafficEnabled(mTrafficCheckbox.isChecked());
      }
  
      public void onTrafficToggled(View view){
          updateTraffic();
      }
  
      private void updateLocation(){
          if(!checkReady()){
              return;
          }
          mGoogleMap.setMyLocationEnabled(mLocationCheckbox.isChecked());
      }
  
      public void onLocationToggled(View view){
          updateLocation();
      }
  
      private void updateBuilding(){
          if(!checkReady()){
              return;
          }
          mGoogleMap.setBuildingsEnabled(mBuildingCheckbox.isChecked());
      }
  
      public void onBuildingToggled(View view){
          updateBuilding();
      }
  
      private void updateIndoor(){
          if(!checkReady()){
              return;
          }
          mGoogleMap.setIndoorEnabled(mIndoorCheckbox.isChecked());
      }
  
      public void onIndoorToggled(View view){
          updateIndoor();
      }
  
      @Override
      public void onItemSelected(AdapterView parent, View view, int position, long id){
           setLayer((String) parent.getItemAtPosition(position));
      }
  
      private void setLayer(String layerName){
          if(!checkReady()){
              return;
          }
  
          if(layerName.equals(getString(R.string.normal))){
              mGoogleMap.setMapType(MAP_TYPE_NORMAL);
          }
          else if(layerName.equals(getString(R.string.hybrid))){
              mGoogleMap.setMapType(MAP_TYPE_HYBRID);
          }
          else if(layerName.equals(getString(R.string.satellite))){
              mGoogleMap.setMapType(MAP_TYPE_SATELLITE);
          }
          else if(layerName.equals(getString(R.string.terrain))){
              mGoogleMap.setMapType(MAP_TYPE_TERRAIN);
          }
          else if(layerName.equals(getString(R.string.none_map))){
              mGoogleMap.setMapType(MAP_TYPE_NONE);
          }
          else{
              Log.i("LDA", "Error setting layer with name " + layerName);
          }
      }
  
      @Override
      public void onNothingSelected(AdapterView parent){
  
      }
  
      @Override
      protected void onResume() {
          super.onResume();
          setUpMapIfNeeded();
          if (mGoogleMap != null) {
              updateTraffic();
              updateLocation();
              updateBuilding();
              updateIndoor();
          }
      }   
  
      @Override
      public boolean onCreateOptionsMenu(Menu menu) {
  
          // Inflate the menu; this adds items to the action bar if it is present.
          getMenuInflater().inflate(R.menu.main, 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();
          if (id == R.id.action_settings) {
              return true;
          }
          return super.onOptionsItemSelected(item);
      }
  }
  </pre>
  
  <p>I've tried to use getSupportFramentManager with android.support.v4.app.DialogFragment support library :</p>
  
  <pre><code>mGoogleMap = ((MapFragment) getSupportFragmentManager().findFragmentById(R.id.map)).getMap();
  </code></pre>
  
  <p>but still doesn't works. I got this error from eclipse <code>Cannot cast from Fragment to MapFragment</code>.
  I've tried to extend to FragmentActivity and still returning null pointer. But when I'm tried to comment getSupportFragmentManager in function setUpMapIfNeeded(), the error was gone.</p>
  
  <p>Here is my LogCat :</p>
  
  <h1><b>Updated</b></h1>
  
  <pre>
  03-24 20:43:36.124: E/AndroidRuntime(27152): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.yai.testmap/com.yai.testmap.MainActivity}: java.lang.NullPointerException
  03-24 20:43:36.124: E/AndroidRuntime(27152):    at com.yai.testmap.MainActivity.setUpMapIfNeeded(MainActivity.java:104)
  03-24 20:43:36.124: E/AndroidRuntime(27152):    at com.yai.testmap.MainActivity.onCreate(MainActivity.java:98)
  </pre>
  
  <p>Any idea?</p>