Tried to create a new world of the "superflat" type, creative, cheats on, with "Overworld" preset. Once I clicked 'Create New World', it shows "Saving World...." and generates a crash report starting with 'NullPointerException: mouseClicked event handler'. Rest can be found in crash log.

It also happens with Tunneler's Dream. Doesn't on default 'superflat'. Assuming it happens on all non-default superflat worlds.