FFAPI can cause Curio items to have a null EquipmentSlot #249

Minecraft version

1.21.1

Describe the bug

When FFAPI is installed, Curio items can sometimes return null for ItemStack#getEquipmentSlot

Steps to reproduce

1. Install FFAPI
2. Install Iron's Spells n' Spellbooks
3. Upgrade a spellbook in an Arcane Anvil using an upgrade orb
4. Crash with NPE

Notes:
According to a dev in the linked issue, getEquipmentSlot is annotated not null:

See:
iron431/irons-spells-n-spellbooks#963

Logs

No response

Additional context

No response