Opera only: Widgetset does not contain impl for null

Hi all,

I know the normal cause of this, but am not sure how to debug in this case. Am working on an app with a logout button that does, among other things, the following:

mainWindow.removeAllComponents();
        removeWindow( <window A> );
        removeWindow( <window B> );
        <window A> = new //etc
        setMainWindow( <window A> );
        this.removeWindow(mainWindow);
        mainWindow = null;

This all works fine on Chrome/FF/Safari, and when the user clicks Logout s/he is brought back to a new login window that we have. But on Opera, I get the following:

Widgetset does not contain implementation for null. Check its @ClientWidget mapping, widgetsets GWT module description file and re-compile your widgetset. In case you have downloaded a vaadin add-on package, you might want to refer to add-on instructions. Unrendered UIDL:
-Unrendered UIDL
-1 id=PID196 width=100.0% margins=0 templateContents=< contents of the html page we use as template >
etc

If I click reload, then the page loads fine. Does anyone know what weirdness Opera is sending to the server that makes this happen? I haven't looked at the traffic on the wire yet; I thought I'd just ask here. I suppose as part of logout I could invalidate the user session, which isn't a bad idea anyway, and that might fix it. Am open to information about the cause though.

I see this with both Opera 10.01 and 11.60.

Thanks,
Bobby