code: public final void addPreferenceFragmentListener(
              @NonNull final PreferenceFragmentListener listener) {
          Condition.INSTANCE.ensureNotNull(listener, "The listener may not be null");
          preferenceFragmentListeners.add(listener);
      }
docs: Adds a new listener, which should be notified, when the currently shown preference fragment
  has been changed, to the activity.
  
  @param listener
  The listener, which should be added, as an instance of the type {@link
  PreferenceFragmentListener}. The listener may not be null