title: NPE in Fragment.setUserVisibleHint() when using ViewPager
body: <p>I am at a loss for this. I am switching tabs manually in my ViewPager. I have this code inside my Activity:</p>
  
  <pre><code>@Override
  public void onBackPressed()
  {
      if (childFragmentStack.empty())
      {
          // Go to the devices screen
          Intent intent = new Intent(this, SelectDeviceActivity.class);
          startActivity(intent);
      }
      else
      {
          Fragment fragment = childFragmentStack.pop();
  
          if (fragment == null)
          {
              return;
          }
  
          processingBackStack = true;
  
          if (fragment instanceof ViewChildFragment)
          {
              viewFragment.activateFragment((ViewChildFragment) fragment);
              mViewPager.setCurrentItem(VIEW_FRAGMENT_INDEX, true);
          }
          else if (fragment instanceof SetupChildFragment)
          {
              setupFragment.activateFragment((SetupChildFragment) fragment);
              mViewPager.setCurrentItem(SETUP_FRAGMENT_INDEX, true); //**
          }
          else if (fragment == homeFragment)
          {
              mViewPager.setCurrentItem(HOME_FRAGMENT_INDEX, true); //**
          }
  
          processingBackStack = false;
      }
  }
  </code></pre>
  
  <p>If I scroll between tabs I add them to a Stack (the 'childFragmentStack'). I am using FragmentPagerAdapter to handle the fragments. What happens is if I do something like View->Setup->View->Setup and then reverse it, it only gets as far as Setup->View->CRASH. It's like when I press Back the Setup Fragment is no longer valid for what I'm doing, but it is never recreated! The Setup fragment is only created in MainActivity.onCreate(), so it should still be around and valid.</p>
  
  <p>The NPE happens on the lines I marked **. Here is the full stack trace:</p>
  
  <pre><code>    04-18 16:04:57.096: E/AndroidRuntime(13072): FATAL EXCEPTION: main
      04-18 16:04:57.096: E/AndroidRuntime(13072): java.lang.NullPointerException
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at android.support.v4.app.Fragment.setUserVisibleHint(Fragment.java:841)
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at android.support.v4.app.FragmentPagerAdapter.setPrimaryItem(FragmentPagerAdapter.java:130)
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at android.support.v4.view.ViewPager.populate(ViewPager.java:1066)
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:550)
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:509)
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at android.support.v4.view.ViewPager.setCurrentItem(ViewPager.java:501)
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at com.lochinvar.serf.MainActivity.onBackPressed(MainActivity.java:234)
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at android.app.Activity.onKeyUp(Activity.java:2131)
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at android.view.KeyEvent.dispatch(KeyEvent.java:2633)
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at android.app.Activity.dispatchKeyEvent(Activity.java:2361)
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:1819)
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at android.view.ViewRootImpl.deliverKeyEventPostIme(ViewRootImpl.java:3577)
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at android.view.ViewRootImpl.handleImeFinishedEvent(ViewRootImpl.java:3547)
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at android.view.ViewRootImpl$ViewRootHandler.handleMessage(ViewRootImpl.java:2797)
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at android.os.Handler.dispatchMessage(Handler.java:99)
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at android.os.Looper.loop(Looper.java:137)
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at android.app.ActivityThread.main(ActivityThread.java:4745)
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at java.lang.reflect.Method.invokeNative(Native Method)
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at java.lang.reflect.Method.invoke(Method.java:511)
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
      04-18 16:04:57.096: E/AndroidRuntime(13072):    at dalvik.system.NativeStart.main(Native Method)
  </code></pre>
  
  <p>[EDIT] I forgot to mention I overrode FragmentPagerAdapter.getPageTitle() and it never returns null (default case goes to a string). </p>