Reproduction Steps

1. Load a world. Yeah, this crash is pretty bad.

Crash Report File

https://gist.github.com/Leo40Git/3ad067e691313a876b0e68a634dbf241

Additional Information

This crash occurred in a development environment. The mod was pulled in via JitPack (so if this isn't repeatable in a production environment, there's the main culprit for a faulty build).

Are you certain you're using the dev JAR? This doesn't reproduce in my environment. It's also worth checking that you're using the correct mappings between both Lithium and your developer workspace.

I'm using build 42 of the mappings vs. your build 14... but Loom should definitely be able to handle that.
I seem to be using the dev JAR? The access widener uses the named namespace. IIRC Loom converts it to intermediary while remapping.

...although, now that I look at the decompiled mixin code, I'm seeing a lot of intermediary names here...

This sounds like SpongePowered/Mixin#431.
This issue was "fixed" in an older version of Fabric's mixin fork but reverted lately due to causing some other error.

As a temporary workaround I would suggest to force the older version containing a fix via adding this in the dependencies block of your buildscript

runtime('net.fabricmc:sponge-mixin:0.8+build.18') {
    force = true
}

Welp, that fixed it. Thanks, PhiPro.
I'm assuming this issue is already open in Fabric's Mixin fork...

No, because it is an upstream Mixin issue. Hence I posted the report on the original Sponge Mixins rather than on Fabric's fork.