Cannot show DialogBox - IllegalStateException

Hi,

what does a "IllegalStateException" mean for a DialogBox?

java.lang.IllegalStateException: This widget's parent does not implement HasWidgets
    at com.google.gwt.user.client.ui.Widget.removeFromParent(Widget.java:204)
    at com.google.gwt.user.client.ui.PopupPanel.show(PopupPanel.java:1016)
    at com.google.gwt.user.client.ui.DialogBox.show(DialogBox.java:432)
    at com.google.gwt.user.client.ui.PopupPanel.center(PopupPanel.java:523)


I'm trying to just show the following custom dialog composite:

MyDialog.java:
class MyDialog extends Composite {
    @UiField
    DialogBox dialog;

    public MyDialog() {
        initWidget(..);
        dialog.show(); //Exception
    }
}

<g:FlowPanel>
  <g:DialogBox ui:field="dialog">
    <g:HTMLPanel>somestuff</g:HTMLPanel>
  </g:DialogBox>
</g:FlowPanel>


MyModule.ui.xml (main module with Entry point):
<my:MyDialog />

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