14 hours ago, Luis_ST said:

take a look into the Item.Properties class, check if there's a method called fireResistant
-> if there is one add the method manually to the Properties
-> if not then look for a method called immuneToFire, (or smoething like this)

I found the other method which this.immuneToFire = true;
btw, how can i add the method manually to the Properties class if this file is read-only?

do not use the auto completion of your IDE