错误描述：
2016-08-18 19:22:46.088 BanTang[3024:197078] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'the view returned from -collectionView:viewForSupplementaryElementOfKind:atIndexPath (UICollectionElementKindSectionFooter, {length = 2, path = 0 - 0}) was not retrieved （取回）by calling -dequeueReusableSupplementaryViewOfKind:withReuseIdentifier:forIndexPath: or is nil ((null))'
