SimplePanel can only contain one child widget - gwt

Question

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
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);
};
}}
});
}}

Answer 1

RootPanel.get() is a SimplePanel. You try to add more than one child to it, resulting in this error.
Instead, you should add HTMLPanel, for example, to your RootPanel, and then add all the children to this HTMLPanel.

Answer 2

I suppose your Diagram extends GWT's SimplePanel
class? SimplePanel can contain only a single widget, have a look here:
http://www.gwtproject.org/javadoc/latest/com/google/gwt/user/client/ui/SimplePanel.html
You could either extend a different class, or call the remove(Widget w) method to remove the existing widget before trying to add a new one.