Wither crashes game

Put the summary of the bug you're having here

What I expected to happen was...:
Kill the Wither in the end dimension

What actually happened was...:
The Game/Server crashes instantly after the Wither had spawned

Steps to Reproduce:
1. Spawned the Wither under the portal in the end
2. Wait until the Wither explodes/spawns
3. Get disconnected/server crashes instantly.