[android-developers] How to dismiss nested dialogfragment correctly after rotation?

I use appcompat library to host some fragments for tabs. In a specific fragment A, I need to do network operation. In order not to block the UI thread, I use an AsyncTask and do the network operation in doInBackGround().

To prevent messing around keep AsyncTask to survive rotation in the fragment, I call setRetainInstance(true) in the fragment A oncreate().

It works fine but I need to prompt user something when the network operation is in progress. Therefore, I use DialogFragment. In the DialogFragment onCreateView(), it instantiates a ProgressDialog and returns.

To be concrete, the following is the code snippets of FragmentA, MyDialogFragment and AsyncTask :

public class FragmentA extends Fragment{ //as tab for actionbar (appcompat)
 private boolean isNetworkOperationCalled;
 private MyDialogFragment myDialogFragment;
 ...
 public void showProgressDialog(){ //for MyAsyncTask#onPreExecute()
  myDialogFragment = new MyDialogFragment();
  myDialogFragment.show(getFragmentManager(), "mydialog");
 }
 public void showProgressDialog(){ //for MyAsyncTask#onPostExecute()
  myDialogFragment.dismiss(); 
 }
 public void onResume() {
  if(!isNetworkOperationCalled){

   isNetworkOperationCalled=true;
   new MyAsyncTask.execute();//do the network operation
  }
 }
 ...
}

public class MyDialogFragment extends DialogFragment{ //ProgressDialog for FragmentA
 ...
 public Dialog onCreateDialog(Bundle savedInstanceState) {
  ProgressDialog progressDialog = new ProgressDialog(getActivity(),ProgressDialog.STYLE_SPINNER);
  ... //set the dialog attributes
  return progressDialog;
 }
 ...
}
public class MyAsyncTask extends AsyncTask<Void,Void,Void>{ //do network operation
 private Fragment fragment;

 public GetTokenAsyncTask(Fragment fragmentA){
  fragment = fragmentA;
 }
 protected void onPreExecute() {
  fragment.showProgressDialog(); //show progess dialog for users
 }
 protected String doInBackground(Void... arg0) {
  ...
 }
 protected void onPostExecute() { //dismiss the dialog
  fragment.dismissProgressDialog();
 }
}
In a nutshell, fragment A triggers AsyncTask and AsyncTask call methods of FragmentA to show/dismiss dialog. Fragment A use MyDialogFragment to show the progress dialog.

The code (method) above works if there is no rotataion occurs while doInBackground() is executing. Nevertheless, if the rotation happens before onPostExecute() is called,
the rotation is fine and the progress dialog is retained. Sadly, it crashes when onPostExecute() finally be called when dismissing the dialog.

E/AndroidRuntime(27493): FATAL EXCEPTION: main
E/AndroidRuntime(27493): java.lang.NullPointerException
E/AndroidRuntime(27493):  at android.support.v4.app.DialogFragment.dismissInternal(DialogFragment.java:184)
E/AndroidRuntime(27493):  at android.support.v4.app.DialogFragment.dismiss(DialogFragment.java:155)
E/AndroidRuntime(27493):  at com.example.FragmentA.dismissProgressDialog(FragmentA.java:105)
E/AndroidRuntime(27493):  at com.example.FragmentA.access$1(FragmentA.java:103)
E/AndroidRuntime(27493):  at com.example.FragmentA$GetTokenAsyncTask.onPostExecute(FragmentA.java:40)
E/AndroidRuntime(27493):  at com.example.FragmentA$GetTokenAsyncTask.onPostExecute(FragmentA.java:1)
E/AndroidRuntime(27493):  at android.os.AsyncTask.finish(AsyncTask.java:631)
E/AndroidRuntime(27493):  at android.os.AsyncTask.access$600(AsyncTask.java:177)
E/AndroidRuntime(27493):  at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
E/AndroidRuntime(27493):  at android.os.Handler.dispatchMessage(Handler.java:99)
E/AndroidRuntime(27493):  at android.os.Looper.loop(Looper.java:137)
E/AndroidRuntime(27493):  at android.app.ActivityThread.main(ActivityThread.java:4875)
E/AndroidRuntime(27493):  at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime(27493):  at java.lang.reflect.Method.invoke(Method.java:511)
E/AndroidRuntime(27493):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:804)
E/AndroidRuntime(27493):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:571)
E/AndroidRuntime(27493):  at dalvik.system.NativeStart.main(Native Method)

How can I solve the problem so that the dialog can dismiss correctly after rotation?

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