Untitled

/Users/mathieumonsauret/Documents/Quiz/MusicQuizDuel/Corona/scripts/ui/SceneFiller.lua:158: attempt to index field 'element' (a userdata value)
stack traceback:
	/Users/mathieumonsauret/Documents/Quiz/MusicQuizDuel/Corona/scripts/ui/SceneFiller.lua:158: in function 'destroy'
	/Users/mathieumonsauret/Documents/Quiz/MusicQuizDuel/Corona/scripts/ui/SpecialGrid.lua:103: in function 'destroy'
	/Users/mathieumonsauret/Documents/Quiz/MusicQuizDuel/Corona/scripts/scenes/choose_category.lua:41: in function </Users/mathieumonsauret/Documents/Quiz/MusicQuizDuel/Corona/scripts/scenes/choose_category.lua:36>
	?: in function 'dispatchEvent'
	?: in function '_listener'
	?: in function <?:141>
	?: in function <?:221>
2015-05-28 10:00:38.178 Corona Simulator[1068:13581] [0m[1m[31m[RUNTIME][0m[0m> Unhandled error: /Users/mathieumonsauret/Documents/Quiz/MusicQuizDuel/Corona/scripts/ui/SceneFiller.lua:158: attempt to index field 'element' (a userdata value)
			[0m[37m[2m[4mmain.lua:20[0m [0m
			[0m[37m[2m[4mui/SceneFiller.lua:158[0m [0m[0m[37m[2m#destroy[0m[0m