If you try to copy and paste something, and you don't have the blocks in your inventory to do it, then click undo... you will get a critical error. I ran into this when I had just opened my game, and forgot to turn creative mode on.
NullPointerException: Exception: REQUESTED TYPE 0 IS NULL
I see that there are some other undo crash reports out there, but my "steps to reproduce" are not the same.