A DESCRIPTION OF THE PROBLEM :
I want to fill menu items while menu on showing, like this:
```
public class Main extends FXApplication {

    @Override
    public void start(Stage primaryStage) throws Exception {
        Menu menu = new Menu("Menu");
        menu.setOnShowing(event -> {
            // Fill menu items.
            menu.getItems().add(new MenuItem("MenuItem 1"));
        });

        MenuBar menuBar = new MenuBar(menu);

        BorderPane borderPane = new BorderPane();
        borderPane.setTop(menuBar);

        Scene scene = new Scene(borderPane);
        primaryStage.setScene(scene);
        primaryStage.show();
    }
}
```
But it cannot work. The reason is that `MenuBarSkin.showMenu` check if the menu is empty and decides whether to show the menu. There are two solutions:
1. Move menu empty check to after the `Menu.ON_SHOWING` or `MenuItem.MENU_VALIDATION_EVENT` event is fired.
2. Add an event before showing the menu.