Activity

private OnServiceStateChangedListener stateChangedListener;
...
if (stateChangedListener != null) { <- Тут null
        Log.d(TAG, "State changed");
        stateChangedListener.onStateChanged(serviceState);
    } else {
        Log.w(TAG, "StateChangedListener is null");
}
...

public void setOnStateChangedListener(OnServiceStateChangedListener listener) {
        if (listener == null) Log.w(TAG, "listener is null"); <- Тут не null
        Log.d(TAG, "StateChangedListener is set");
        stateChangedListener = listener;
    }


public void removeStateChangedListener() { stateChangedListener = null; }


public interface OnServiceStateChangedListener {
    void onStateChanged(State state);
}
--------------------------------------------------------------------------------------

Fragment
private MainActivity.OnServiceStateChangedListener stateChangedListener;

...

@Override
public void onResume() {
    super.onResume();

    MainActivity activity = (MainActivity)getActivity();
    if (activity != null) {
        Log.d(TAG, "Setting stateChangedListener");
        createStateChangedListener();
        activity.setOnStateChangedListener(stateChangedListener);
    } else {
        Log.w(TAG, "Can't set stateChangedListener. Activity is null");
    }
}

private void createStateChangedListener() {
    stateChangedListener = new MainActivity.OnServiceStateChangedListener() {
        ...
    }
}


Log:
D/HomeFragment: Setting stateChangedListener
D/MainActivity: StateChangedListener is set
W/MainActivity: StateChangedListener is null