ContentLayout not showing

[blay]

Hello:

I have this class

public class ButtonExample extends LayoutContainer{

    public void onRender(Element parent, int index){
        super.onRender(parent, index);
        
        Button button= new Button("CLICK ME!",new SelectionListener<ButtonEvent>(){  
            public void componentSelected(ButtonEvent ce) {  
                MessageBox box = new MessageBox();
                box.setButtons(MessageBox.OKCANCEL);
                box.setTitle("Botón pulsado");
                box.show();
            }
        });
        
        add(button);
    }
}

Then I have this other class:

public class MainMenu extends Viewport{

    private ButtonExample buttonExample = null;
    
    @Override
    protected void onRender(com.google.gwt.user.client.Element parent, int index) {
        super.onRender(parent, index);
    
        //String xml = MenuConstants.stringXML;
        
        final MenuBar bar = new MenuBar();
        final ContentPanel mainPanel = new ContentPanel();
        Menu menu;
        MenuItem menuItem;
        MenuBarItem menuBarItem;
        
        menu = new Menu();
        menuItem = new MenuItem("Alta");   
        menuItem.addSelectionListener(new SelectionListener<MenuEvent>(){

            @Override
            public void componentSelected(MenuEvent ce) {
            
                //setReferencesNull();
                buttonExample = new ButtonExample();
                mainPanel.add(buttonExample);
            }
            
        });
        
        menu.add(menuItem);

        menuBarItem = new MenuBarItem("Empresas Grupo",menu);
        bar.add(menuBarItem);
        
mainPanel.setTopComponent(bar);
        add(mainPanel);
        
    }

If I add the buttonExample in the onRender body, it works, and a button appears. But if I add it on the SelectionListener, it doesn´t appear. I´m missing something, any help?

[sven]

 menuItem.addSelectionListener(new SelectionListener<MenuEvent>(){

            @Override
            public void componentSelected(MenuEvent ce) {
            
                //setReferencesNull();
                buttonExample = new ButtonExample();
                mainPanel.add(buttonExample);
mainPanel.layout();
            }
            
        });

Everytime you fnished with modifying a container, you need to call layout.