title: App crashes when adding a fragment
body: <p>I tried to create a simple color change application. In my application I have created a fragment which has linear layout and inside which text view is present. I am trying to change the color of textview in my application in a regular interval. In order to change color in regular interval, I have used random() function to create a random number between 0 and 255 and use those numbers as a parameter to create random colors. I am again using those random colors to set the background of my textview which is present in linear layout.</p>
  
  <p>Below are my codes:</p>
  
  <p>MainActivity.java</p>
  
  <pre><code>package com.example.abcd.color_change;
  
  import android.support.v4.app.Fragment;
  import android.support.v4.app.FragmentManager;
  import android.support.v7.app.AppCompatActivity;
  import android.os.Bundle;
  
  public class MainActivity extends AppCompatActivity {
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
  
          FragmentManager fragment = getSupportFragmentManager();
          Fragment existingFragment = fragment.findFragmentById(R.id.container);
          fragment.beginTransaction().replace(R.id.linear1, existingFragment).commit();
  
  
      }
  }
  </code></pre>
  
  <p>ColorChangeFragment.java</p>
  
  <pre><code>package com.example.abcd.color_change;
  
  import android.graphics.Color;
  import android.os.Bundle;
  import android.view.LayoutInflater;
  import android.view.View;
  import android.view.ViewGroup;
  import android.widget.LinearLayout;
  import android.widget.TextView;
  
  import java.util.Random;
  import java.util.Timer;
  import java.util.TimerTask;
  
  /**
   * Created by abcd on 10/7/17.
   */
  
  public class ColorChangeFragment {
      private TextView mTextView;
      private Random rand = new Random();
      private int randNum = 0;
  
      public View onCreateView(LayoutInflater inflater, ViewGroup container,Bundle savedInstanceState){
  
          View v = inflater.inflate(R.layout.fragment,container, false);
          mTextView = (TextView) v.findViewById(R.id.textview);
  
          Timer timer = new Timer();
  
          //MyTimer mt = new MyTimer();
          /*timer.schedule(mt, 1000, 1000);*/
          timer.schedule(new TimerTask(){
  
              @Override
              public void run() {
                  int r = rand.nextInt(255);
                  int g = rand.nextInt(255);
                  int b = rand.nextInt(255);
                  int randomColor = Color.rgb(r,g,b);
                  mTextView.setBackgroundColor(randomColor);
              }
          },5000,5000);
          return v;
      }
  
     /* class MyTimer extends TimerTask{
  
          @Override
          public void run() {
              runOnUiThread(new Runnable(){
  
                  @Override
                  public void run() {
                      Random rand = new Random();
                      mTextView.setBackgroundColor(Color.argb(255, rand.nextInt(256), rand.nextInt(256) ));
                  }
              });
          }
      }*/
  
  
  }
  </code></pre>
  
  <p>ActivityMain.xml</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;FrameLayout
      xmlns:android="http://schemas.android.com/apk/res/android"
      android:id="@+id/container"
      android:layout_width="match_parent"
      android:layout_height="match_parent"&gt;
  
  &lt;/FrameLayout&gt;
  </code></pre>
  
  <p>fragment.xml</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
      android:orientation="vertical" android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:id ="@+id/linear1"
      android:background="@android:color/white"&gt;
  
      &lt;TextView
          android:layout_width="match_parent"
          android:layout_height="match_parent"
          android:id = "@+id/textview"/&gt;
  
  &lt;/LinearLayout&gt;
  </code></pre>
  
  <p>Below is the error message I receive upon building the code.</p>
  
  <pre><code>10-30 12:56:21.393 21574-21574/? E/AndroidRuntime: FATAL EXCEPTION: main
                                                     Process: com.example.abcd.color_change, PID: 21574
                                                     java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.abcd.color_change/com.example.abcd.color_change.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Class java.lang.Object.getClass()' on a null object reference
                                                         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2762)
                                                         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2848)
                                                         at android.app.ActivityThread.-wrap12(ActivityThread.java)
                                                         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1552)
                                                         at android.os.Handler.dispatchMessage(Handler.java:102)
                                                         at android.os.Looper.loop(Looper.java:154)
                                                         at android.app.ActivityThread.main(ActivityThread.java:6334)
                                                         at java.lang.reflect.Method.invoke(Native Method)
                                                         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
                                                         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
                                                      Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Class java.lang.Object.getClass()' on a null object reference
                                                         at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:394)
                                                         at android.support.v4.app.BackStackRecord.replace(BackStackRecord.java:441)
                                                         at android.support.v4.app.BackStackRecord.replace(BackStackRecord.java:432)
                                                         at com.example.abcd.color_change.MainActivity.onCreate(MainActivity.java:17)
                                                         at android.app.Activity.performCreate(Activity.java:6743)
                                                         at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1134)
                                                         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2715)
                                                         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2848) 
                                                         at android.app.ActivityThread.-wrap12(ActivityThread.java) 
                                                         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1552) 
                                                         at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                         at android.os.Looper.loop(Looper.java:154) 
                                                         at android.app.ActivityThread.main(ActivityThread.java:6334) 
                                                         at java.lang.reflect.Method.invoke(Native Method) 
                                                         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
                                                         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) 
  10-30 12:56:21.393 21574-21574/? D/AppTracker: App Event: crash
  </code></pre>