setting up the window.

AppDelegateMethod.swift

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
// Override point for customization after application launch.
//New code starts from here
window = UIWindow(frame: UIScreen.main.bounds)
window?.makeKeyAndVisible()


let mainVC = ViewController()
window?.rootViewController = mainVC


return true
}