public ObjectProperty<ItemSortingTopic> itemSortingTopicProperty() {
        if (null == itemSortingTopic) {
            itemSortingTopic = new ObjectPropertyBase<ItemSortingTopic>(_itemSortingTopic) {
                @Override protected void invalidated() { fireTileEvent(DATA_EVENT); }
                @Override public Object getBean() { return Tile.this; }
                @Override public String getName() { return "itemSortingTopic"; }
            };
            _itemSortingTopic = null;
        }
        return itemSortingTopic;
    }
Defines an optional node that can be used in combination with the
CustomTileSkin.
@param GRAPHIC
src/main/java/eu/hansolo/tilesfx/Tile.java