Apple has a warning for the users who are willing to update,
Using Xcode 11 beta 3 with macOS Catalina beta 4 will prevent SwiftUI previews from working. Update to Xcode 11 beta 4 when it becomes available.
The build can be downloaded by via the Apple Developer Center, or as an over-the-air update.
As with all new betas, macOS Catalina should not be installed on a primary machine because it is early release software and has bugs.