Unsure whether to post this here or the other mod's github, but noticed there's a compatibility issue where a non-recoverable error that gets thrown when attempting to open the GUI from the RPG mod by Gerkiz. I've included what the pop up says below:
The mod RPG Items (0.21.15) caused a non-recoverable error.
Please report this error to the mod author.
Error while running event m-rpg_items::on_entity_died (ID 4)
m-rpg_items/control.lua:711: attempt to index field 'rpgitems_item_gui' (a nil value)
stack traceback:
m-rpg_items/control.lua:711: in function <m-rpg_items/control.lua:692>