Just to get a little bit more context, what are you trying to do? Have the character use the consumable item from the InventoryPanel byt equipping it, use it, and unequip it just like we do in the demo scene with the Fire ball?
From your setup I don't see anything wrong, so perhaps the issue lies somewhere else. Check that you are specifying the correct ItemCollections for equipping, The CharacterUseItemAction uses the index of Equippable ItemCollections I believe.
As for the error, it seems to be related to the inspector. It is not something that should be happening.
Could you explain how to reproduce those errors? Is it at edit time and/or runtime? does it always happens, when does it happen? How can I reproduce the error in the demo scene? I've never seen this one before