Incompatibility with SkinsDB

Crashes whenever you have a gun in your hand with this error:

AsyncErr: Lua: Runtime error from mod 'guns4d' in callback environment_Step(): ...4/.minetest/mods/guns4d/classes/Player_model_handler.lua:307: attempt to concatenate local 'fallback' (a nil value)
stack traceback:
	...4/.minetest/mods/guns4d/classes/Player_model_handler.lua:307: in function '_construct_low'
	/home/miner34/.minetest/mods/leef_class/new_class.lua:50: in function 'construct'
	/home/miner34/.minetest/mods/leef_class/new_class.lua:47: in function 'construct'
	/home/miner34/.minetest/mods/leef_class/new_class.lua:91: in function 'new'
	...miner34/.minetest/mods/guns4d/classes/Player_handler.lua:41: in function 'update'
	/home/miner34/.minetest/mods/guns4d/init.lua:278: in function </home/miner34/.minetest/mods/guns4d/init.lua:269>
	/usr/share/luanti/builtin/common/register.lua:26: in function </usr/share/luanti/builtin/common/register.lua:12>