title: NPE in FragmentManagerImpl.popBackStackImmediate()
body: <p>When I quickly press back, back, back on my mobile I got following exception:</p>
  
  <pre><code>java.lang.NullPointerException
  at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1378)
  at android.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:437)
  at android.app.FragmentManagerImpl.popBackStackImmediate(FragmentManager.java:452)
  at android.app.Activity.onBackPressed(Activity.java:2123)
  at com.swipemaster.activities.CommonActivity.onBackPressed(CommonActivity.java:16)
  at com.swipemaster.activities.MainActivity.access$0(MainActivity.java:1)
  at com.swipemaster.activities.MainActivity$1.run(MainActivity.java:62)
  at android.os.Handler.handleCallback(Handler.java:605)
  at android.os.Handler.dispatchMessage(Handler.java:92)
  at android.os.Looper.loop(Looper.java:137)
  at android.app.ActivityThread.main(ActivityThread.java:4424)
  at java.lang.reflect.Method.invokeNative(Native Method)
  at java.lang.reflect.Method.invoke(Method.java:511)
  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:817)
  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:584)
  at dalvik.system.NativeStart.main(Native Method)
  </code></pre>
  
  <p>Tested on Android 4.0.4 and 2 devices: Sony Xperia MT27i and ST23i. On 2.2.1 and Galaxy Mini this problem doesn't occur. Can anybody help me?</p>
  
  <p><code>CommonActivity</code> class:</p>
  
  <pre><code>public class CommonActivity extends Activity
  {
      @Override
      public void onBackPressed()
      {
          MusicPlayer.continueMusic = true;
          super.onBackPressed(); // this is CommonActivity.java:16
      }
  </code></pre>