Handle missing mcPool gracefully in spriteSheet buildMovieClip (#6196)
Handle missing mcPool gracefully in spriteSheet buildMovieClip (#6196)

Prevents an issue with freezing game dev levels.
javatlacati/codecombat,javatlacati/codecombat,codecombat/codecombat,codecombat/codecombat,codecombat/codecombat,javatlacati/codecombat,javatlacati/codecombat,javatlacati/codecombat,codecombat/codecombat,codecombat/codecombat