There is what I believe to be a very severe design error in the Main.htm in the Gizmox.WebGUI.Forms.2.0 project under Commons/Dialog.
You can get the same error (on the resize event) if you resize the browser while refreshing.
I believe the problem lies with the "deferred" setting in the line:
I am not 100% sure on this but from what I could quickly gather...
The solution has been for me to remove that deferred attribute or comment out all of the events except for onload and a few others that I can't seem to get to break.
I also don't believe you should be wiring up those events on the body tag. I have always been told that the onload event is the only safe event to begin running code in. And from what I have experienced with VWG, I would say that this is true. I did try to wire those events inside of the onload event but it didn't quite work for some reason.
I had been working with the source and at some point someone had put in checks to see if the functions tied to the events on the body tag were available...but by the time the code made it to a build, those checks were removed. I don't believe that was a good fix either. I think the events need to be wired in during the onload event.