This project demonstrates a UICollectionView crash when using performBatchUpdates and in the updates implicitly remove a header. The crash can be observed by running the application, and removing the last item via bar button "Delete". Alternatively the UITests can be run which will trigger the crash immediately. The problem stems from the code in collectionView(_:layout:referenceSizeForHeaderInSection:) which returns CGSize.zero in case there are no items. So when deleting the last item in performBatchUpdates, the UICollectionView will also call that delegate method, and when the header changes from a positive height to 0, it will crash on Xcode 9 on iOS 11.4 simulator, or freeze for 15 seconds (!!) on Xcode 10 beta 6 on iOS 12 simulator.
The issue seems to be fixed in iOS 12.1.