It's not an insta crash, it crashes after the world is kinda "loaded" or while its loading.

Crash: https://gist.github.com/Fundryi/2e170e4cdd4bc099191e960b3bccc82e

O! This error has already met, but I could not figure out what is the reason.
If possible, send me somehow saving the world with this error. This will help me solve it.

Please check the latest version MistWorld_1.12.1_alpha_a_05.jar
It seems to me, I was able to fix the bug.