This related error was in the log: https://pastebin.com/58n4Fcfd

Crucibles don't accept goo from any direction. I was using plant goo specifically in this case. This is in version 1.16.1-0.6.2.7. (edit: and 0.6.3.5)

I think the problem is that updateFluidsTo tries to setGoo(fluids.get(0)) without checking if fluids.get(0) exists first.

I'll eventually get a Forge environment set up so I can directly help instead of opening a bunch of issues :)

Fixed already.