I haven't been able to identify a mod conflict, but I'm also not the most code literate. 1.3.9 Gives me no issue.
https://gist.githubusercontent.com/Cloak-Bot/ff63ce51153d288b724a5c98b7991c30/raw/21ef2150eda839489fef1596609628d38da149c0/crash.txt

I tried prod with carpet only and fabric API. Seems like some of the mods is cutting that branch of code off. Since its related to world generation, that might be world edit, or maybe some of the lithium optimizations. If you can try to identify the mod conflict I might be able to look it up and fix it, but seems like it might end up as won't fix

Alright, I was able to deduce it's a conflict with optifine, optifabric, or both after running a clean installation with only fabric, fabric extra, optifabric/optifine. Is it possible I've accidentally got an optifine setting on that does't play well with carpet? I am able to join a server, singleplayer exclusively gives me the crash.

@gnembon
This is caused due to a conflict with Optifine and Optifabric.
It was reported to the author of Optifabric here:[https://github.com/modmuss50/OptiFabric/issues/145]
and he seems to have identified the issue, as for the fix, I don't know.

not able to fix - original optifine removed vanilla class that is needed. I guess behaviour with optifine of any mods is non-deterministic as optifine bypasses mixins that fabric has a control on.
Will fix the error, but this means that chunk generation will not be detectable in scarpet when running fabric, at least for now

will not crash next release I guess

Thank you for all the hard work you do for us, it's very much appreciated! Does this mean I should close the issue?

Running carpet alongside optifine will make so that scarpet apps won't be able to plug in to the chunk generation event. Otherwise it should work just fine.