fix crash when setting layoutSpec's child to nil
fix crash when setting layoutSpec's child to nil

flovouin/AsyncDisplayKit,Xinchi/AsyncDisplayKit,rahul-malik/AsyncDisplayKit,smyrgl/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,samhsiung/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,harryworld/AsyncDisplayKit,rmls/AsyncDisplayKit,romyilano/AsyncDisplayKit,levi/AsyncDisplayKit,jellenbogen/AsyncDisplayKit,garrettmoon/AsyncDisplayKit,hannahmbanana/AsyncDisplayKit,garrettmoon/AsyncDisplayKit,maicki/AsyncDisplayKit,garrettmoon/AsyncDisplayKit,smyrgl/AsyncDisplayKit,harryworld/AsyncDisplayKit,Xinchi/AsyncDisplayKit,Adlai-Holler/AsyncDisplayKit,JetZou/AsyncDisplayKit,gazreese/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,gazreese/AsyncDisplayKit,samhsiung/AsyncDisplayKit,JetZou/AsyncDisplayKit,Xinchi/AsyncDisplayKit,yufenglv/AsyncDisplayKit,romyilano/AsyncDisplayKit,bimawa/AsyncDisplayKit,rmls/AsyncDisplayKit,george-gw/AsyncDisplayKit,garrettmoon/AsyncDisplayKit,bimawa/AsyncDisplayKit,rmls/AsyncDisplayKit,levi/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,yufenglv/AsyncDisplayKit,yufenglv/AsyncDisplayKit,samhsiung/AsyncDisplayKit,hannahmbanana/AsyncDisplayKit,lappp9/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,samhsiung/AsyncDisplayKit,chrisdanford/AsyncDisplayKit,rahul-malik/AsyncDisplayKit,george-gw/AsyncDisplayKit,bimawa/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,JetZou/AsyncDisplayKit,chrisdanford/AsyncDisplayKit,JetZou/AsyncDisplayKit,lappp9/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,flovouin/AsyncDisplayKit,chrisdanford/AsyncDisplayKit,bimawa/AsyncDisplayKit,romyilano/AsyncDisplayKit,maicki/AsyncDisplayKit,Adlai-Holler/AsyncDisplayKit,levi/AsyncDisplayKit,hannahmbanana/AsyncDisplayKit,flovouin/AsyncDisplayKit,yufenglv/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,harryworld/AsyncDisplayKit,flovouin/AsyncDisplayKit,jellenbogen/AsyncDisplayKit,Xinchi/AsyncDisplayKit,smyrgl/AsyncDisplayKit,smyrgl/AsyncDisplayKit,Adlai-Holler/AsyncDisplayKit,levi/AsyncDisplayKit,harryworld/AsyncDisplayKit,hannahmbanana/AsyncDisplayKit,rahul-malik/AsyncDisplayKit,jellenbogen/AsyncDisplayKit,rmls/AsyncDisplayKit,romyilano/AsyncDisplayKit,lappp9/AsyncDisplayKit,jellenbogen/AsyncDisplayKit,Adlai-Holler/AsyncDisplayKit,facebook/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,george-gw/AsyncDisplayKit,maicki/AsyncDisplayKit,facebook/AsyncDisplayKit,JetZou/AsyncDisplayKit,chrisdanford/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,lappp9/AsyncDisplayKit,flovouin/AsyncDisplayKit,maicki/AsyncDisplayKit,levi/AsyncDisplayKit,gazreese/AsyncDisplayKit,facebook/AsyncDisplayKit,facebook/AsyncDisplayKit,maicki/AsyncDisplayKit,gazreese/AsyncDisplayKit,yufenglv/AsyncDisplayKit,jellenbogen/AsyncDisplayKit,hannahmbanana/AsyncDisplayKit,Xinchi/AsyncDisplayKit,rmls/AsyncDisplayKit,george-gw/AsyncDisplayKit,harryworld/AsyncDisplayKit,Adlai-Holler/AsyncDisplayKit,rahul-malik/AsyncDisplayKit,garrettmoon/AsyncDisplayKit