Hi,
I finished chapter 17 and the app crashes because window is nil.
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
// Override point for customization after application launch.
let navigationController = window!.rootViewController as! UINavigationController
let controller = navigationController.viewControllers[0] as! AllListsViewController
controller.dataModel = dataModel
return true
}