Can't attach debugger to UNNotificationServiceExtension
Don't hit run on Xcode when the debugger is attached to the service extension.
Connect the debugger to the Containing app and run the app. Then attach the debugger to the service extension.
I don't know why you can't run it when the debugger is connected to the service extension. I'm guessing it has something to do with NSExtensionPointIdentifier set to com.apple.usernotifications.service and once the OS sees that things go to hell because only the OS should launch the service extension.