GWT Designer only allows one root GUI object per class.
The code generated by the Google plugin is not very well factored, and has two root GUI objects... the rootpanel and the dialogbox.
A the moment, GWT Designer just chooses the root with the most children (in this case the dialog box).
We will probably change this to always give the rootpanel precedence.
The correct way to handle this example is to refactor it and split the dialogbox out into its own class.
Note that you can use code hiding tags
to hide the dialogbox and its children. That will cause the rootpanel to appear in the design view.