I'm using 0.5-beta.
...ar\Addons\ForgeUI_Inventory\ForgeUI_InventoryBag.lua:177: attempt to call field 'API_RegisterAddon' (a nil value) stack trace: ...ar\Addons\ForgeUI_Inventory\ForgeUI_InventoryBag.lua:177: in function <...ar\Addons\ForgeUI_Inventory\ForgeUI_InventoryBag.lua:170>

Inventory is not in v0.5 yet. Just delete it for the moment.