I absolutely love this mod, it's great. I know the 1.12 version is still very much WIP, but that crash may help you avoid coding problems: The game crashed whilst ticking entity
The game crashed whilst ticking entity
Error: java.lang.NullPointerException: Ticking entity I've got no idea what entity does this, but I'm 100% sure it's from this mod. Everything else works fine though, and I'm looking forward to see the full version.
That's very limited info. Are you using the most recent release? I would need to see a crashlog and normal log of that session to be able to determine what's going on. The current version on its own is quite stable; so if anything crashes it is most likely due to some interaction with another mod.
Also, if you want to use it, this is where I collect bugs/issues: https://github.com/Daveyx0/PrimitiveMobs/issues
Thanks for playing and reporting!