Forge v14.23.4.2707-1.12.2
Yoyos v1.2.2.20
Metamorph v1.1.6
I got this NPE when I hit a Metamorph Entity with the Rainbow Yoyo. No crash occurred.
Latest.log

It actually looks like More Player Models is what's causing the NPE. The mod is closed source, so not sure how much I can do here.

I went ahead and reported it to that mod as well. Maybe the mod author will take a look at it.
Fixed link: https://www.curseforge.com/minecraft/mc-mods/more-player-models/issues/50