[Request] Fix for Underdone RP

I’ve tried for few days to fix few problems I am having with the Underdone files, but I can’t find a way to fix it.
The gamemode I am using can be found here.

At the moment when I hold Q to get to my inventory, it gives me error.

vgui.Create: Error when calling 'inventorytab':Init (...one\gamemode\core\clientfiles\menutabs\inventory.lua:132: attempt to concatenate field '?' (a nil value))
ERROR: GAMEMODE:'OnSpawnMenuOpen' Failed: ...done\gamemode\core\clientfiles\menus\cl_mainmenu.lua:59: attempt to index field 'InventoryTab' (a nil value)

How can this be fixed? Its weird that in singleplayer it works fine, but in Multiplayer it gives this.

Are you using rebelland as the map?