seems like qsl is putting an immutable collection in a place porting lib doesn't expect. unsure why. https://mclo.gs/BAYOb4D

with certain perf mods, this crash is delayed until the create world sreen is opened.

I believe this issue is occurring due to not having the Fabric API mod installed, which makes the relevant collection mutable.

n/a, that means there's an incompatibility with qfapi, as it's the replacement on quilt - reported there as qfapi#177. worth leaving open, you'll just get duplicates otherwise.