I've been working on an interface using GXT3 and UiBinder, when I found a strange bug. I used ButtonGroup inside a toolbar in order to group buttons and to make the toolbar look better. This worked fine as long as I was debugging the application, but when I buildt a version for deployment, the application crashed at startup. I traced this back to when I added the ButtonGroup elements. When I removed these, the application run perfectly in its deploy version.
Thanks for the update on this - no changes were made to ButtonGroup since before 3.0.1, nor its superclass, SimpleContainer, which also is the base class for many other containers. HBoxLayoutContainer did have some changes, mostly involved around when the container is not given an explicit size. This could be the case in your code, since you elements won't get a size (except for width on the HBoxLC) - the Grid is stopping explicit height from passing down from the ButtonGroup.
That's about as far as I can speculate without knowing what you mean by 'crashing' (exception? did nothing?), and more details about how to run this and what it should look like/do.