I guessed it would be caused by that, and I know it’s bad practice to use spaces or characters in internal variable names but either the characters shouldn’t be allowed or the issue should be fixed as it could cause some confusion as to “why has my editor just crashed?” or “why can’t I add stuff to my button?”
Of everything in Unreal Engine 4.7 right now I’d have to say that UMG is probably the least stable although it does work!