Fabric Loader: 0.4.4 build 138
Yarn: 1.14 build 3

How to reproduce: Start server, connect to it with client...

https://gyazo.com/1dc8cf103975cd9bdfb1a1ef7d4f761c

Thanks for the report. I believe I've already identified the cause of this in passing-- it should be fixed rather shortly in the next patch.

Next patch soonish? Just curious :)

Absolutely- I'm building the new version as I type this!

Hang on for a second, I may have jumped to conclusions. It looks like I can't actually reproduce this on my end (my apologies, I could have sworn I tested this, but it must have been related to a different issue). At least in a development environment, a client can successfully join a server with Stockpile 1.0.1.

What other mods are present in your environment? Does removing Stockpile prevent the error from occurring?

Removing stockpile solves it

Have you tried outside of dev?

Just did. I was able to connect to a server with just Fabric API 0.2.7 build 127 and Stockpile 1.0.1. Perhaps the issue is coming from a conflict rather than a bug in Stockpile on its own.

Here's a test build of 1.0.2 if you want to see if the problem persists with it as well. Version 1.0.2 does not bundle the Kotlin language adapter so you will need to download it manually if you haven't already.
stockpile-1.0.2-server-npe-test+1.14.zip

As i did some more testing it seems like it was Composable Automation causing the crash on login. Sorry for bothering you!

Ah, that's totally fine. No worries-- thanks for taking the time to contribute regardless!