title: app crashes after exiting?
body: <p>I am making an app in which multiple HorizontalScrollview ,Overall the app runs well but when the user exit Boom app crashes , logcast tells me that it has multiple markers in onPostExecute :</p>
  
  <pre><code>rootView = inflater.inflate(R.layout.three_fragment, container, false);
      L1= (LinearLayout) rootView.findViewById(R.id.LinearLayout1);
      L3= (TextView) rootView.findViewById(R.id.CheckTest);
      prog =(ProgressBar)rootView.findViewById(R.id.progressBar);
  
  
  
          booksList = new ArrayList&lt;Books&gt;();
  
  
  
          new JSONAsyncTask().execute("http://192.168.30.3/actors/book_tab2.php");    
  
          centerLockHorizontalScrollview = (CenterLockHorizontalScrollview) rootView.findViewById(R.id.HMscrollView); 
  
            adapter = new CardHomeAdapter(getActivity(), R.layout.row_home_card, booksList);
  
  
          centerLockHorizontalScrollview.setAdapter(TabThreeFragment.this, adapter); 
  
              rootView.setLayoutParams(new FrameLayout.LayoutParams(FrameLayout.LayoutParams.MATCH_PARENT,FrameLayout.LayoutParams.MATCH_PARENT ));   
  
  
              }   
          }
  </code></pre>
  
  <p>onPostExecute :</p>
  
  <pre><code>protected void onPostExecute(Boolean result) {
  
      adapter.notifyDataSetChanged();
  
      if(result == false)
  
          Toast.makeText(getActivity(), "Unable to fetch data from server", Toast.LENGTH_LONG).show(); 
      if(booksList != null) {
  
  
          adapter = new CardHomeAdapter(getActivity().getApplicationContext(), R.layout.row_home_card, booksList);  
         centerLockHorizontalScrollview.setAdapter(TabThreeFragment.this, adapter); 
      }
  </code></pre>
  
  <p>Error :</p>
  
  <pre><code>03-07 11:49:21.478: E/AndroidRuntime(9703): FATAL EXCEPTION: main
      03-07 11:49:21.478: E/AndroidRuntime(9703): java.lang.NullPointerException
      03-07 11:49:21.478: E/AndroidRuntime(9703):     at br.liveo.fragments.TabThreeFragment$JSONAsyncTask.onPostExecute(TabThreeFragment.java:344)
      03-07 11:49:21.478: E/AndroidRuntime(9703):     at br.liveo.fragments.TabThreeFragment$JSONAsyncTask.onPostExecute(TabThreeFragment.java:1)
      03-07 11:49:21.478: E/AndroidRuntime(9703):     at android.os.AsyncTask.finish(AsyncTask.java:631)
      03-07 11:49:21.478: E/AndroidRuntime(9703):     at android.os.AsyncTask.access$600(AsyncTask.java:177)
      03-07 11:49:21.478: E/AndroidRuntime(9703):     at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
      03-07 11:49:21.478: E/AndroidRuntime(9703):     at android.os.Handler.dispatchMessage(Handler.java:99)
      03-07 11:49:21.478: E/AndroidRuntime(9703):     at android.os.Looper.loop(Looper.java:137)
      03-07 11:49:21.478: E/AndroidRuntime(9703):     at android.app.ActivityThread.main(ActivityThread.java:4961)
      03-07 11:49:21.478: E/AndroidRuntime(9703):     at java.lang.reflect.Method.invokeNative(Native Method)
      03-07 11:49:21.478: E/AndroidRuntime(9703):     at java.lang.reflect.Method.invoke(Method.java:511)
      03-07 11:49:21.478: E/AndroidRuntime(9703):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1004)
      03-07 11:49:21.478: E/AndroidRuntime(9703):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:771)
      03-07 11:49:21.478: E/AndroidRuntime(9703):     at dalvik.system.NativeStart.main(Native Method)
  </code></pre>