Interface between activity and fragment becomes null automatically

I have an activity with a menu button and two buttons. I have three fragments Frag_A, Frag_B, and Frag_C. By default, I am loading the activity with the fragment Frag_A. Based on clicking different buttons in the activity, I switch between the fragments.

For the fragment to access the activity buttons, I have created an interface BasicClickListener and I have made the activity implement the interface.

public interface BasicClickListeners {

    void onMenuClick();

    void goToFragment2();

    void goToFragment3();
} 

I have implemented two methods in all fragments:

public synchronized void registerBasicListener(BasicClickListeners listener) {
    basicClickListeners = listener;
}

private synchronized void unregisterBasicListener() {
    basicClickListeners = null;
}

So, whenever I launch a fragment through the activity, I call registerBasicListener() method from the activity. And on all fragments' onDestroy(), I am calling unregisterBasicListener.

@Override
    public void onDestroy() {
        super.onDestroy();
        unregisterBasicListener();
    }

Now if I wanted to go to fragment 2 from fragment 1, I use basiClickListeners.goToFragment2() from fragment 2, and it works.

I don't know why sometimes I am getting NPE on basicClickListeners. Even though the fragment is active, how can the basicClickListeners become null? And it's not happening always. Only sometimes. I couldn't identify a pattern on why it's occurring. Kindly assist me with this issue.

You can use onAttach() and onDetach() overridden method in Fragment for registering and clearing the interface listeners. Also, make sure that the interface is implemented in your activity. Then, you can use mCallback reference for calling the goToFragment2(), goToFragment3() methods.

YourFragment

public class YourFragment extends Fragment {
    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        try {
            mCallback = (BasicClickListeners) context;
        } catch (ClassCastException e) {
            throw new ClassCastException(context.toString()
                    + " must implement BasicClickListeners");
        }
    }
    @Override
    public void onDetach() {
        mCallback = null;
        super.onDetach();
    }
}

YourActivity:

public class MainActivity extends AppCompatActivity 
implements BasicClickListeners {

}