Fix crash when menu view controller is being set to nil, close #28
Fix crash when menu view controller is being set to nil, close #28

yky138495/REFrostedViewController,mobitar/REFrostedViewController,renmoqiqi/REFrostedViewController,dinhchitrung/REFrostedViewController,romaonthego/REFrostedViewController,mohsinalimat/REFrostedViewController,BlessNeo/REFrostedViewController,tangwei6423471/REFrostedViewController,js/REFrostedViewController,AlexanderMazaletskiy/REFrostedViewController,ywzw2013/REFrostedViewController,rayshen/REFrostedViewController,githubdelegate/REFrostedViewController,jkereako/REFrostedViewController,imjerrybao/REFrostedViewController,ianisme/REFrostedViewController,giuseppenucifora/REFrostedViewController,ernestopino/REFrostedViewController,takeshineshiro/REFrostedViewController,shuketa/REFrostedViewController,TaemoonCho/REFrostedViewController,HqRaining78/REFrostedViewController