fix iOS 6 crash
fix iOS 6 crash

bobqian1130/TWMessageBarManager,VojtaStavik/TWMessageBarManager,hbucius/TWMessageBarManager,revile/TWMessageBarManager,primoz990/TWMessageBarManager,terryworona/TWMessageBarManager,AlexanderMazaletskiy/TWMessageBarManager,cooksimo/TWMessageBarManager,zuojx1013/TWMessageBarManager,ben181231/TWMessageBarManager,xueqiu/TWMessageBarManager,CharlyInc/TWMessageBarManager,howeguo/TWMessageBarManager