title: NullPointerException when showing a ViewPager a second time
body: <p>I am using a <code>ViewPager</code> fragment which has two fragments as children. This works great, however when I replace the <code>ViewPager</code> fragment by another fragment and replace this fragment by the <code>ViewPager</code> fragment my application crashes with the following <code>NullPointerException</code>:</p>
  
  <pre><code>java.lang.NullPointerException: Attempt to invoke virtual method 'int java.util.ArrayList.size()' on a null object reference
      at android.support.v4.app.FragmentManagerImpl.getFragment(FragmentManager.java:667)
      at android.support.v4.app.FragmentStatePagerAdapter.restoreState(FragmentStatePagerAdapter.java:211)
      at android.support.v4.view.ViewPager.onRestoreInstanceState(ViewPager.java:1319)
      at android.view.View.dispatchRestoreInstanceState(View.java:13756)
      at android.view.ViewGroup.dispatchRestoreInstanceState(ViewGroup.java:2888)
      at android.view.View.restoreHierarchyState(View.java:13734)
      at android.support.v4.app.Fragment.restoreViewState(Fragment.java:468)
      at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1094)
      at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1248)
      at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:738)
      at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1613)
      at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:517)
      at android.os.Handler.handleCallback(Handler.java:739)
      at android.os.Handler.dispatchMessage(Handler.java:95)
      at android.os.Looper.loop(Looper.java:135)
      at android.app.ActivityThread.main(ActivityThread.java:5294)
      at java.lang.reflect.Method.invoke(Native Method)
      at java.lang.reflect.Method.invoke(Method.java:372)
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904)
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:699)
  </code></pre>
  
  <p>My <code>ViewPager</code> fragment consists of the following source code instanciating the two child fragments in case they are null. Furthermore, the <code>ViewPagerAdapter</code> implementation is instanciated and assigned to the <code>ViewPager</code>.</p>
  
  <pre><code>public class ConnectionPasswordViewPagerFragment extends Fragment
  {
      private ViewPager vpConnectionPassword;
      private ConnectionPasswordViewPagerAdapter paConnectionPassword;
      private ConnectionPasswordGeneratorMACAddress passwordGeneratorMACAddress;
      private ConnectionPasswordGeneratorSerialNumber passwordGeneratorSerialNumber;
  
      public ConnectionPasswordViewPagerFragment()
      {
      // Required empty public constructor
      }
  
      @Override
      public void onCreate(Bundle savedInstanceState)
      {
      super.onCreate(savedInstanceState);
  
      if ((this.passwordGeneratorSerialNumber == null) &amp;&amp; (this.passwordGeneratorMACAddress == null))
      {
          this.passwordGeneratorMACAddress = new ConnectionPasswordGeneratorMACAddress();
          this.passwordGeneratorSerialNumber = new ConnectionPasswordGeneratorSerialNumber();
      }
      }
  
      @Override
      public View onCreateView(LayoutInflater inflater, ViewGroup container,
                           Bundle savedInstanceState)
      {
      // Inflate the layout for this fragment
      return inflater.inflate(R.layout.fragment_connection_password_view_pager, container, false);
      }
  
      @Override
      public void onActivityCreated(Bundle savedInstanceState)
      {
      super.onActivityCreated(savedInstanceState);
  
      /*if (this.paConnectionPassword == null)
      {*/
          this.paConnectionPassword = new ConnectionPasswordViewPagerAdapter(this.getChildFragmentManager(), this.getActivity(), this.passwordGeneratorMACAddress, this.passwordGeneratorSerialNumber);
      //}
      this.vpConnectionPassword = (ViewPager) this.getActivity().findViewById(R.id.vpConnectionPassword);
      this.vpConnectionPassword.setAdapter(this.paConnectionPassword);
      }
  }
  </code></pre>
  
  <p>The <code>ViewPagerAdapter</code> shown below implementation keeps the reference to the child fragments in an <code>ArrayList</code> and implements the metods <code>getItem()</code>, <code>getCount()</code> as well as <code>getPageTitle()</code>. The class <code>ViewPagerFragment</code> simply extends the support <code>Fragment</code> class and provides the abstract method <code>getPageTitleStringID()</code> implemented by the child fragments.</p>
  
  <pre><code>public class ConnectionPasswordViewPagerAdapter extends FragmentStatePagerAdapter
  {
      private static final byte NUMBER_OF_FRAGMENTS = (byte) 2;
  
      private ArrayList&lt;ViewPagerFragment&gt; childFragments;
      private Activity displayActivity;
  
      public ConnectionPasswordViewPagerAdapter(FragmentManager fragmentMgm, Activity displayAct, ConnectionPasswordGeneratorMACAddress generatorMACAddress, ConnectionPasswordGeneratorSerialNumber generatorSerialNumber)
      {
      super(fragmentMgm);
  
      this.childFragments = new ArrayList&lt;&gt;(ConnectionPasswordViewPagerAdapter.NUMBER_OF_FRAGMENTS);
      this.setDisplayActivity(displayAct);
      if (generatorMACAddress != null)
      {
          this.childFragments.add(generatorMACAddress);
      }
      if (generatorSerialNumber != null)
      {
          this.childFragments.add(generatorSerialNumber);
      }
      }
  
      public Activity getDisplayActivity()
      {
      return this.displayActivity;
      }
  
      public void setDisplayActivity(Activity value)
      {
      this.displayActivity = value;
      }
  
      @Override
      public Fragment getItem(int position)
      {
      return this.childFragments.get(position);
      }
  
      @Override
      public int getCount()
      {
      return ConnectionPasswordViewPagerAdapter.NUMBER_OF_FRAGMENTS;
      }
  
      @Override
      public CharSequence getPageTitle(int position)
      {
      return this.getDisplayActivity().getString(this.childFragments.get(position).getPageTitleStringID());
      }
  }
  </code></pre>
  
  <p>Thanks for your help!</p>