requestAuthorization intermittently throws

The following code intermittently throws causing app to crash;


center.requestAuthorization(options:[.badge, .alert, .sound]) { (granted, error) in
 }

it is called from within func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool method.

center is defined as;


let center = UNUserNotificationCenter.current()


Any clue as to why center.requestAuthorization intermittently throws causing crash?


Thanks