title: SimplePanel can only contain one child widget
body: <p>When i run the following Code and push Button "Push", more than once a time, i get browser error "simplePanel can only contain one child widget". How can i solve that problem? thank you in advance! Jogi</p>
  
  <pre><code>public class Projekt implements EntryPoint {
  
      private RootPanel rootPanel;
  
      public void onModuleLoad() {
  
          rootPanel = RootPanel.get("gwtContainer");
          rootPanel.setSize("1902", "868");
  
           final AbsolutePanel boundaryPanel = new AbsolutePanel();
           boundaryPanel.setStyleName("frame1");
           boundaryPanel.setSize("1455px", "600px");       
  
           final Diagram diagram = new Diagram(boundaryPanel);
  
           RootPanel.get().add(boundaryPanel, 446, 242);
  
  
           final Connector con = new Connector(100, 300, 300, 500);
  
           Button la = new Button("Push");
           la.setSize("200", "200");
           RootPanel.get().add(la);
  
           Button la2 = new Button("Push2");
           la2.setSize("200", "200");
           RootPanel.get().add(la2);
  
           final Image img = new Image("images/concrete.svg");
           img.setSize("200", "200");
  
  
           final Shape shapei = new Shape(img);
  
  
           Image img2 = new Image("images/variable.svg");
           img2.setSize("200", "200");
           boundaryPanel.add(img2, 200,200);
  
           final Shape shapei2 = new Shape(img2);
           shapei2.showOnDiagram(diagram);
  
           la.addClickHandler(new ClickHandler(){
  
              @Override
              public void onClick(ClickEvent event) {
                   boundaryPanel.add(img, 100,100);
                   shapei.showOnDiagram(diagram);
              }
  
           });
           la2.addClickHandler(new ClickHandler(){
  
                  @Override
                  public void onClick(ClickEvent event) {
  
                       con.showOnDiagram(diagram);
                  }
  
               });
  
  
           diagram.addDiagramListener(new DiagramListenerAdapter() {
  
                @Override
                public void onElementConnect(ElementConnectEvent event) {
                  if (con.startEndPoint.isGluedToConnectionPoint()) {
                      Widget connected = con.startEndPoint.gluedConnectionPoint.parentWidget;
                      if(connected.equals(shapei.connectedWidget)){
                    Image logo = new Image("images/xor.svg");
                    logo.setSize("100", "100");
                    boundaryPanel.add(logo);
                  }
                      else if(connected.equals(shapei2.connectedWidget)){
                           Image logo2 = new Image("images/and.svg");
                        logo2.setSize("100", "100");
                        boundaryPanel.add(logo2);
                      };
                }}
              });
  
  }}
  </code></pre>