My app crashes when it tries to display an error message from AsyncTask when it runs in background

I am displaying an error message from an AsyncTask to my Fragment but when my app is on background the app is crashing.

the code is below:

import android.support.v4.app.FragmentManager;

public class AppUtil {

    public static void showErrorDialog(FragmentManager fm, int messageResourceId) {
        if (fm != null && !fm.isDestroyed()) {
            InfoDialogFragment.newInstance("", messageResourceId).show(fm, "ERROR");
        }
    }

    public static boolean isEmpty(String s) {
        return s == null || s.trim().length() == 0;
    }
}


import android.app.ProgressDialog;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

public class BlankFragment extends Fragment  implements ILoginable, 
View.OnClickListener {

     private ProgressDialog pd ;
    public BlankFragment() {
        // Required empty public constructor
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View rootView =  inflater.inflate(R.layout.fragment_blank, 
     container, false);
        rootView.findViewById(R.id.pressBT).setOnClickListener(this);
        return rootView;
    }

    @Override
    public void starProgress(int messageId) {
        if (pd == null) {
            pd = new ProgressDialog(getContext(), R.string.loading);
            pd.show();
        }
    }

    @Override
    public void endProgress() {
        if(pd != null && isAdded()) {
            pd.dismiss();
            pd = null;
        }
    }

    @Override
    public void displayError(int messageId) {
        if (isAdded()) {
            AppUtil.showErrorDialog(getFragmentManager(), R.string.app_name);
        }
    }

    @Override
    public void loginResult(boolean success) {

    }

    @Override
    public void onClick(View v) {
        if (R.id.pressBT == v.getId()) {
            new LoginAsyncTask(this).execute("x");
        }
    }
}


public interface ILoginable {
    void starProgress(int messageId);
    void endProgress();
    void displayError(int messageId);
    void loginResult(boolean success);
}


import android.os.AsyncTask;

import java.lang.ref.WeakReference;

public class LoginAsyncTask extends AsyncTask<String, Void, String> {

    private WeakReference<ILoginable> reference;

    public LoginAsyncTask(ILoginable iLoginable) {
        reference = new WeakReference<>(iLoginable);
    }

    @Override
    protected void onPreExecute() {
        reference.get().starProgress(R.string.loading);
    }

    @Override
    protected String doInBackground(String... strings) {
        return "xx";
    }

    @Override
    protected void onPostExecute(String s) {
        if (reference.get() !=  null) {
            reference.get().endProgress();
            reference.get().displayError(R.string.hello_blank_fragment);
        }
        reference.clear();
    }
}

As you can see, this is a simple behavior for a basic asyncTask. However I can not figure out why the isAdded method returns true when the app is on background and also why the app is crashing with the message:

*** Process: com.roscasend.android.testadd, PID: 2891
java.lang.IllegalStateException: Can not perform this action after onSaveInstanceState
    at android.support.v4.app.FragmentManagerImpl.checkStateLoss(FragmentManager.java:2053)
    at android.support.v4.app.FragmentManagerImpl.enqueueAction(FragmentManager.java:2079)
    at android.support.v4.app.BackStackRecord.commitInternal(BackStackRecord.java:678)
    at android.support.v4.app.BackStackRecord.commit(BackStackRecord.java:632)
    at android.support.v4.app.DialogFragment.show(DialogFragment.java:143)
    at com.roscasend.android.testadd.AppUtil.showErrorDialog(AppUtil.java:17)
    at com.roscasend.android.testadd.BlankFragment.displayError(BlankFragment.java:51)
    at com.roscasend.android.testadd.LoginAsyncTask.onPostExecute(LoginAsyncTask.java:37)
    at com.roscasend.android.testadd.LoginAsyncTask.onPostExecute(LoginAsyncTask.java:15)***

Best Regards, Aurelian

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