Library conflict in MacOS Catalina - UIFoundation vs AppKit
macOS Catalina adds a lot of under the hood components to better support iOS on Mac apps. Both UIKit and AppKit are installed with macOS 10.15.

I haven't extensively tested the Catalina beta, but my best answer right now would be that it's still an early beta (beta 3 at the time of writing) and errors like this are to be expected. 

If you are testing an Xcode project on Catalina, I would assume that your app will (mostly) work correctly, but you should file a bug report if you come across any problems that you think may be a bug in the operating system or SDK.