code: public final void addNavigationListener(@NonNull final NavigationListener listener) {
          Condition.INSTANCE.ensureNotNull(listener, "The listener may not be null");
          navigationListeners.add(listener);
      }
docs: Adds a new listener, which should be notified, when a navigation preference has been added or
  removed to/from the activity.
  
  @param listener
  The listener, which should be added, as an instance of the type {@link
  NavigationListener}. The listener may not be null