Accessing buttons on a UIActivityViewController in an iOS 13 UI test causes a crash

I'm seeing this behavior with Xcode 11 too, and I think your assumption that it's an Xcode bug is correct.

The crash appears to be fixed with Xcode Version 11.2 beta 2 (11B44), but querying for buttons in a UIActivityViewController is still broken. (I never see the element resolve.) Hopefully Apple will fix element lookup in the near future.