https://pastebin.com/9wP69rVi

I don't know. I spawned a boss from Box of horrors and then opened the creative menu. It then crashed and gave me this error report. Showing you mod.

The issue looks weird, but I'll look closer when I'm back, I'm not home for now. Thank you for reporting it

Could you please try 0.1.6 version of the mod and look if the issue is fixed, I made some changed in classes mentioned in report