This pops up every time i change anything in "Settings > Mod Settiongs > Startup" even if if the changes aren't supposed to affect the "RPG Items" mod

The mod RPG Items (0.21.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event m-rpg_items::on_configuration_changed
m-rpg_items/control.lua:257: attempt to index field 'm-spell-pack' (a nil value)
stack traceback:
m-rpg_items/control.lua:257: in function <m-rpg_items/control.lua:157>

It seems simple, I'll fix it soon.

I published a new version. I hope, it'll fix it.

Looks like you broke something else.

841.474 Error MainLoop.cpp:1284: Exception at tick 23940: The mod RPG Items (0.21.2) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event m-rpg_items::on_nth_tick(60)
m-rpg_items/control.lua:523: attempt to index field 'rpgitems_item_gui' (a nil value)
stack traceback:
m-rpg_items/control.lua:523: in function <m-rpg_items/control.lua:499>
841.474 Error ServerMultiplayerManager.cpp:92: MultiplayerManager failed: "The mod RPG Items (0.21.2) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event m-rpg_items::on_nth_tick(60)
m-rpg_items/control.lua:523: attempt to index field 'rpgitems_item_gui' (a nil value)
stack traceback:
m-rpg_items/control.lua:523: in function <m-rpg_items/control.lua:499>"

After some more testing, looks like a conflict with Gerkiz's diablo-like RPG system. It crashes upon opening the gui for that mod.

https://github.com/ZwerOxotnik/rpg_items/issues/2

I made a new fix for it, It should work now. https://mods.factorio.com/mod/m-rpg_items/downloads