What's the issue you encountered?

More information about the issue:
AlexModGuy/Rats#823 (comment)

ok, turns out you just need serene seasons, snowy spirit, and rats to replicate this.
It looks like Serene Seasons isn't checking that the ClientLevel isn't null before fetching the dimension in getClientSeasonState. Snowy spirit calls for this in their seasons compat when determining if its Christmas season. In rats, I build a cache of all Mobs on level load for the first time to use for the mob filter using LevelEvent.Load. For some reason brains are constructed in the entity's constructor, causing the code Snow Spirit injects into villagers for Serene Seasons compat to fire. This event fires just before the ClientLevel is assigned to the game, so everything crashes and burns.
I would report this to Serene Seasons. You're more than welcome to link this issue to it, and if I got anything wrong, I'm sure someone will correct me.

How can the issue be reproduced?

Start minecraft with serene seasons, snowy spirit, and rats (rats should be from github, not the version from curseforge).
World loading will freeze at 100% forever.

Logs

https://drive.google.com/file/d/104qzNy1cQZ4Tdd7KtehYntoUEYCazJx7/view?usp=sharing

Mod Version

9.0.0.43, 46 as well

Additional information

Rats-1.20.1-8.1-BETA-3.zip
Rats version to replicate (I hope it's okay to send the compiled version here).