Step to reproduce.
- Creating Superflat World (I've only tested with Void type.)
- and the game suddenly crashed.
It also happen with the old world (pre-8 and below.)
This seems to be related to MobSpawning.
A Temporary fix is using a programm like https://irath96.github.io/webNBT/ to edit the level.dat, and set the gamerule "doMobSpawning" to "false".