I installed macOS Catalina Beta 3 today and also the latest Xcode 11 beta. I launched Xcode 11, opened the FreelanceStation project, and then hit Cmd+B to build it. The build failed with an error indicating Xcode couldn’t open a particular storyboard. I first thought the new Xcode was the problem, but it also happened with version 10. So I opened the storyboard as source code and started cutting pieces of it until Xcode was able to open it in IB mode.
The failing storyboard contained an NSCollectionView with an NSCollectionViewFlowLayout which, for some odd reason, was missing a size property. I compared it with other storyboards that had NSCollectionView and they surely had that property, so I included it in the offending storyboard and IB was able to open it. I don’t know how that happened, but it seems macOS Catalina requires that property to be there in order to instantiate an NSCollectionViewFlowLayout.