The game crashed when I did the following steps, although I have not been able to reproduce the crash:

1. Built an End portal
2. Killed the Enderdragon
3. Used an End gateway
4. Used the other gateway to get back

At the time, I thought this was due to unloading the end portal, so I tried to unload it again in a new world. I couldn't get it to crash, but the same error appears in the log as soon as the Enderdragon is killed.

Game log (from testing afterwards): https://paste.ee/p/u9G7b
Crash log: https://paste.ee/p/KAjtQ

Both of these logs were with CTM and Twilight Forest added, but the error still appears in the log when they are removed.

Edit: These errors seem to go away in the log after quitting and re-opening the world.