fixed error in ' dlg_create_world.lua'

this error comes up, well it did now crazyginger72 fixed it :)

02:48:39: ERROR[main]: MAINMENU ERROR: ...cal/share/minetest/builtin/mainmenu/dlg_create_world.lua:89: bad argument #2 to 'setting_set' (string expected, got nil)
02:48:39: ERROR[main]: stack traceback:
02:48:39: ERROR[main]: 	[C]: in function 'setting_set'
02:48:39: ERROR[main]: 	...cal/share/minetest/builtin/mainmenu/dlg_create_world.lua:89: in function 'handle_buttons'
02:48:39: ERROR[main]: 	/usr/local/share/minetest/builtin/fstk/ui.lua:118: in function 'handle_buttons'
02:48:39: ERROR[main]: 	/usr/local/share/minetest/builtin/fstk/ui.lua:155: in function </usr/local/share/minetest/builtin/fstk/ui.lua:148>