title: How to add close button in GWT
body: <p>I've this code:</p>
  
  <pre><code>...
  MenuBar options = new MenuBar(true);
  options.addItem("First Label", new ScheduledCommand()
          {
          @Override
              public void execute() {
  
              PopupPanel popupProperties = new PopupPanel();
              TabPanel tabPanel = new TabPanel();
              FlowPanel flowpanel = new FlowPanel();
              ...
              flowPanel.add(...);
              scroll = new ScrollPanel(flowPanel); 
              tabPanel.add(scroll, "First Tab");
              popupProperties.add(tabPanel);
  
              RootPanel.get().add(popupProperties);
              popupProperties.center();
              popupProperties.show();
              }
          }
  ...
  </code></pre>
  
  <p>If I add SimplePanel with Button (close) to popupProperties, it doesn't work. How can I do it? Thanks a lot.</p>