title: ViewPager PagerObserver was not registered
body: <p>My application closes when i turn off my screen and turn it back on.</p>
  
  <p>Logcat tells me that the cause of this error is pointing to java line 60 in my CourseFragment.class which extends Fragment. </p>
  
  <pre><code>mViewPager.setAdapter(infoTechPageAdapter);
  </code></pre>
  
  <p>I am implementing a ViewPager in my CourseFragment.class which extends Fragment.</p>
  
  <p>Here's my code:</p>
  
  <pre><code>public class CourseFragment extends Fragment {
  
  public static final String ARG_POSITION_NUMBER = "course_number";
  
  private int position;
  
  public CourseFragment() {
      // Empty constructor required for fragment subclasses
  }
  
  @Override
  public View onCreateView(LayoutInflater inflater, ViewGroup container,
          Bundle savedInstanceState) {
  
      View rootView;
      position = getArguments().getInt("position");
  
      String course = getResources().getStringArray(
              R.array.array_navigation_drawer)[position];
  
      getActivity().setTitle(course);
  
      rootView = inflater.inflate(R.layout.activity_drawer_fragment,
              container, false);
  
      return rootView;
  }// end onCreateView
  
  @Override
  public void onStart() {
      super.onStart();
  
      ViewPager mViewPager = (ViewPager) getActivity().findViewById(
              R.id.pagerYear);
  
      Log.i("posit", String.valueOf(position));
      switch (position) {
  
      case 0:
          InfoTechPageAdapter infoTechPageAdapter = new InfoTechPageAdapter(
                  getActivity().getSupportFragmentManager());
          mViewPager.setAdapter(infoTechPageAdapter);
          break;
  
      case 1:
          ComSciPageAdapter comSciPageAdapter = new ComSciPageAdapter(
                  getActivity().getSupportFragmentManager());
          mViewPager.setAdapter(comSciPageAdapter);
          break;
  
      case 2:
          ProfilePageAdapter profilePageAdapter = new ProfilePageAdapter(
                  getActivity().getSupportFragmentManager());
          mViewPager.setAdapter(profilePageAdapter);
          break;
      }
  
  }// end onStart
  }
  </code></pre>
  
  <p>Logcat</p>
  
  <pre><code>09-22 10:54:03.830: E/AndroidRuntime(2917): FATAL EXCEPTION: main
  09-22 10:54:03.830: E/AndroidRuntime(2917): java.lang.IllegalStateException: Observer android.support.v4.view.ViewPager$PagerObserver@4239b3e8 was not registered.
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.database.Observable.unregisterObserver(Observable.java:69)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.support.v4.view.PagerAdapter.unregisterDataSetObserver(PagerAdapter.java:294)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at com.usjr.sss.adapter.InfoTechPageAdapter.unregisterDataSetObserver(InfoTechPageAdapter.java:21)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.support.v4.view.ViewPager.setAdapter(ViewPager.java:409)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at com.usjr.sss.fragment.CourseFragment.onStart(CourseFragment.java:60)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.support.v4.app.Fragment.performStart(Fragment.java:1502)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:957)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1104)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:682)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1460)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:472)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.support.v4.app.FragmentStatePagerAdapter.finishUpdate(FragmentStatePagerAdapter.java:163)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.support.v4.view.ViewPager.setAdapter(ViewPager.java:415)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at com.usjr.sss.fragment.CourseFragment.onStart(CourseFragment.java:60)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.support.v4.app.Fragment.performStart(Fragment.java:1502)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:957)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1104)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1086)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.support.v4.app.FragmentManagerImpl.dispatchStart(FragmentManager.java:1882)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:573)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1164)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.app.Activity.performStart(Activity.java:5114)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.app.Activity.performRestart(Activity.java:5169)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.app.ActivityThread.handleSleeping(ActivityThread.java:3408)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.app.ActivityThread.access$2700(ActivityThread.java:153)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1426)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.os.Handler.dispatchMessage(Handler.java:99)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.os.Looper.loop(Looper.java:137)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at android.app.ActivityThread.main(ActivityThread.java:5227)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at java.lang.reflect.Method.invokeNative(Native Method)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at java.lang.reflect.Method.invoke(Method.java:511)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:795)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:562)
  09-22 10:54:03.830: E/AndroidRuntime(2917):     at dalvik.system.NativeStart.main(Native Method)
  </code></pre>