code: public final void addRestoreDefaultsListener(@NonNull final RestoreDefaultsListener listener) {
          Condition.INSTANCE.ensureNotNull(listener, "The listener may not be null");
          this.restoreDefaultsListeners.add(listener);
      }
docs: Adds a new listener, which should be notified, when the preferences' default values should be
  restored, to the fragment.
  
  @param listener
  The listener, which should be added as an instance of the type {@link
  RestoreDefaultsListener}. The listener may not be null