Crash Dump: https://pastebin.com/JUVuZ2ZM

When typing to spawn a Fox Using
Entity e = et.spawn(iuc.getWorld(), new CompoundTag(), null, null, iuc.getBlockPos(), SpawnType.EVENT,false,false);

Causes a Crash

Well ,this is an NPE. Maybe one of these nulls or empty tags is causing an issue?

That's possible, but those 2 are for TextComponent for Custom Name and PlayerEntity, and it also works on all other mobs(to my knowledge)

I tried passing non-null arguments into though spots, but still NPE

This doesn't seem like a fabric api issue to me, but rather how you are spawning them. Vanilla spawns them just fine.