title: FragmentTransaction is throwing null reference
body: <p>I have loader fragment and I am calling another  fragment from the <code>loader fragment</code>.Everything is working fine but suddenly my fragment loader is giving me an error.In the loader fragment I am displaying progress bar for 5sec and after that another fragment will display.</p>
  
  <p>Here is my code:-</p>
  
  <p><strong>loader fragment.java</strong></p>
  
  <pre><code>package com.example.user.attendance;
  public class Loader_fragment extends Fragment {
      AVLoadingIndicatorView Loader;
      Runnable runnable;
      Handler handler;
      FragmentManager fragmentManager;
      FragmentTransaction fragmentTransaction;
      Timer timer;
  
  
      @Override
      public View onCreateView(LayoutInflater inflater, ViewGroup container,
                               Bundle savedInstanceState) {
          // Inflate the layout for this fragment
          View loadder_fragment=inflater.inflate(R.layout.fragment_loader_fragment, container, false);
  
          Loader=(AVLoadingIndicatorView)loadder_fragment.findViewById(R.id.loder_login);
          handler=new Handler();
          runnable=new Runnable() {
              @Override
              public void run() {
                  Loader.setVisibility(View.INVISIBLE);
                  timer.cancel();
                  fragmentManager=getFragmentManager();
                  fragmentTransaction=fragmentManager.beginTransaction();
                  fragmentTransaction.setCustomAnimations(R.animator.slide_in_left,R.animator
                          .slide_out_left,R.animator.slide_in_right,R.animator.slide_out_right);
                  Home_Screen home_screen=new Home_Screen();
                  fragmentTransaction.replace(R.id.Home_screen_fragment,home_screen);
                  fragmentTransaction.addToBackStack(null);
                  fragmentTransaction.commit();
  
              }
  
  
          };
  
          timer=new Timer();
          timer.schedule(new TimerTask() {
              @Override
              public void run() {
                  handler.post(runnable);
              }
          }, 5000,5000);
  
  
         /* if(Loader.setVisibility(View.INVISIBLE)==1){
  
  
          }*/
  
          return loadder_fragment;
      }
  
  
  }
  </code></pre>
  
  <p><strong>Logcat is showing an error:-</strong></p>
  
  <pre><code> Process: com.example.user.attendance, PID: 1200
      java.lang.NullPointerException: Attempt to invoke virtual method 'android.app.FragmentTransaction android.app.FragmentManager.beginTransaction()' on a null object reference
          at com.example.user.attendance.Loader_fragment$1.run(Loader_fragment.java:44)
          at android.os.Handler.handleCallback(Handler.java:739)
          at android.os.Handler.dispatchMessage(Handler.java:95)
          at android.os.Looper.loop(Looper.java:148)
          at android.app.ActivityThread.main(ActivityThread.java:7402)
          at java.lang.reflect.Method.invoke(Native Method)
          at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
  </code></pre>
  
  <p>Why it is showing null object reference in the fragment.</p>