Not sure which mod it is as Vanilla fix says either Apotheosis or Mob Grinding Utils, but I am getting a null pointer with the mob grinder from that mod with Sharpness X and Looting X. After a few kills or immediately the game crashes with this report. I posted on Mob Grinding Utils github, so they are aware as well.

https://paste.dimdev.org/sohemikite.mccrash

MC 1.12.2
Forge 2815