code: public final void addAreaListener(@NonNull final AreaListener listener) {
          Condition.INSTANCE.ensureNotNull(listener, "The listener may not be null");
          this.areaListeners.add(listener);
      }
docs: Adds a new listener, which should be notified, when an area is modified by the dialog.
  
  @param listener
  The listener, which should be added, as an instance of the type {@link AreaListener}.
  The listener may not be null