public StringProperty alertMessageProperty() {
        if (null == alertMessage) {
            alertMessage = new StringPropertyBase(_alertMessage) {
                @Override protected void invalidated() { fireTileEvent(ALERT_EVENT); }
                @Override public Object getBean() { return Tile.this; }
                @Override public String getName() { return "alertMessage"; }
            };
            _alertMessage = null;
        }
        return alertMessage;
    }
Defines the text that could be used in a tooltip as an
alert message.
@param MESSAGE
src/main/java/eu/hansolo/tilesfx/Tile.java