Hi,
From what I see on images you attached slot2 and slot5 have no script connected.
Method defined in inventory is:
func slot_gui_input(event : InputEvent, slot : SlotClass)
It is expecting slot
of type SlotClass
, but I guess that if your slots (2 and 5) don't have script connected they are not of that type and because of that slot
in above method is being null
.