When I build my Xcode app (play button) It does not work anymore and I see this :
Fortunately I can use the preview.
Xcode bug
Your app most likely has crashed.
To see where the app is crashing, create an exception breakpoint in Xcode by choosing Debug > Breakpoints > Create Exception Breakpoint. If the app crashes Xcode will pause the app where the crash occurs. The following article may help:
https://www.swiftdevjournal.com/fixing-and-avoiding-crashes-in-swift-code/