my world crashed

Put the summary of the bug you're having here

What I expected to happen was...:
Describe what you thought should happen here
i should spawn on my sky city i was building with a wither skeleton i spawned with a command block should start attacking me
What actually happened was...:
Describe what happened here
the game crashed when i logged on to my world
Steps to Reproduce:
1. i spawned a wither skeleton
2. it turned to black when i spawned
3. then the game crashed