this crashes

[errant badge]

Can you show your code for that entity?

[restive sapphire]

you have a null stack in RyuHeadEntity somewhere

ItemStack are supposed to be never null

[humble plank]

idk what do you mean by null itemstack

[restive sapphire]

wdym you don't know what a null itemstack is?

that code is full of null stacks

[humble plank]

i mean, idk on what to return for example in getArmorItems?

[restive sapphire]

to say that there are no armour bits around?

[humble plank]

okay, that's easy, but i can't understand what to add in getMainArm()

[restive sapphire]

you return one of the values of the Arm enum

and fix getEquippedStack too

(that's what crashing you)