This is related to the mod Neat. Viewing a Trackman villager will immediately crash the game. These can be spawned in with the mob egg (and some luck) to test.

Crash log: https://pastebin.com/1fyYQRwS

For players: This can be temporarily worked around by adding "Villager" as an entry to the Neat blacklist.

@liach This one might concern you

Dupe of #1167. Medium priority because the game crashes.

Ah, so if I understand correctly the problem was that items being disabled would cause the villager to crash when the GUI was opened, and Neat simply exposed that information to the player sooner. Is that right?

When removing Neat I found I couldn't actually open the trading GUI, but there may be additional checks in place there.

Thanks for looking into it in regardless.

Dunno what caused the other behavior, but I've fixed a crash of the same stacktrace.