Getting a freeze and error log when moving equipped items from their slot to an empty area in the equipment UI. Moving it to the inventory moves it correctly, moving it to the game world drops it correctly, moving it from the equipped slot to an incompatible slot gives the correct error message, and moving it from the inventory to an empty spot in the equipment UI drops it. 

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ERROR in

action number 1

of Mouse Event for Glob Left Released

for object EquipmentSlot:

Unable to find instance for object index -4

at gml_Object_EquipmentSlot_Mouse_56

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gml_Object_EquipmentSlot_Mouse_56 (line -1)