I'm not entirely sure if what I experienced is the same as your own crash, but I was able to reproduce an issue with the same symptoms as yours. Would you be able to confirm the issue I'm experiencing below is the same thing as what happened to you, share a crash report from this happening, and if your issue is different explain in more detail exactly how it can be reproduced?
I placed down a trainer npc and set their model to "Custom Playername", entered my own username into the box (which correctly displayed my skin on the trainer upon leaving the menu). I then, with the value still in the box, changed the model type to "Custom Resource Pack", and crashed a few seconds afterwards. I have linked the crash report this generated here.
I've moved the information I gathered so far to internal to be looked into, but confirmation/further information would help in getting this resolved. Thanks!