All you need to do is expand the inner window and move the manifest part of the bar into the empty space under "Create Prefab" (or, turn the Manifest bar off entirely.)
Depends on how often it's crashing. It crashes for everyone now and then, and the more complex the map gets, the more likely to crash it is. If it's doing it really often (like, every few minutes) it's probably a problem with your machine.
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