Describe the bug
Upon trying to create a world or join an existing world, the game closes out instantly, crashing. There is likely an incompatibility with a mod in my custom modpack, but this crash only occurs on Sophisticated Backpacks versions 3.15.0.493 up to the current 3.15.12.552 version. If you are unable to get enough information to solve this issue from my crash logs, I can try to narrow down the incompatibility on my end, just let me know.

Every time I crash, it generates both a server and a client crash log.
Server Crash Log: https://pastebin.com/tyDtUBFt
Client Crash Log: https://pastebin.com/DuQLAP7j

To Reproduce
Steps to reproduce the behavior:
Since this only happens in my (currently private) modpack, it won't normally be replicable. But this is what I do for it to happen:
1: Create a new world/Join existing world
2: World will begin to generate
3: As soon as player tries to load into the world, game will crash instantly before actually joining

Expected behavior
I expect the game not to crash when trying to join a new or existing world.

Versions
The latest version of Sophisticated Backpacks I can use without crashes is 1.16.5-3.14.3.484
Forge Version: 36.2.31
Minecraft Version: 1.16.5

Oops, my bad. I was using an outdated version of polymorph which was causing this issue.