Show alert after app start - ios

Question

I have a problem with showing alert after application start. I am using Notification Center observe notification error and find current visible view controller to show alert on it. This work when I handle error after start an app, but if I want to show this error just after start an app, it does not happen.
My code for app delegate:
func listenForRealmErrorNotification() {
NotificationCenter.default.addObserver(forName: MyDataModelDidFailNotification, object: nil, queue: OperationQueue.main, using: { notification in
let alert = UIAlertController(title: NSLocalizedString("Internal alert", comment: "Internal error header"),
message: NSLocalizedString("There was an error while working whith your data", comment: "Internal error description") + "\n\n" + NSLocalizedString("Press OK to terminate the app. Sorry for the inconvenience", comment: "Internal error excuses"),
preferredStyle: .alert)
let action = UIAlertAction(title: NSLocalizedString("OK", comment: "Agree button on internal error"), style: .default, handler: {_ in
let exeption = NSException(name: NSExceptionName.internalInconsistencyException, reason: "Realm error", userInfo: nil)
exeption.raise()
})
alert.addAction(action)
print("***Observe error")
self.viewControllerForShowingAlert().present(alert, animated: true, completion: nil)
})
}
func viewControllerForShowingAlert() -> UIViewController {
let rootViewController = self.window!.rootViewController!
return topViewController(from: rootViewController)
}
func topViewController(from controller: UIViewController) -> UIViewController {
if controller is UINavigationController {
return topViewController(from: (controller as! UINavigationController).visibleViewController!)
}
if controller is UITabBarController {
return topViewController(from:(controller as! UITabBarController).selectedViewController!)
}
if let presentedViewController = controller.presentedViewController {
return topViewController(from:presentedViewController)
}
return controller
}
And code for post notification:
func fatalRealmError(_ error: Error) {
print("***Fatal error with dataBase: \(error)")
Crashlytics.sharedInstance().recordError(error)
NotificationCenter.default.post(name: MyDataModelDidFailNotification, object: nil)
}
UPDATE:
Initiating my data source in delegate:
func initialDataSource() {
do {
dataSource = try UserDataSource()
}
catch let error as NSError {
fatalRealmError(error)
}
}
And here I have set am observer:
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
customizeAppearance()
listenForRealmErrorNotification()
initialDataSource()
let rootViewController = window?.rootViewController as! UINavigationController
let rootContentController = rootViewController.viewControllers[0] as! YourFoodViewController
rootContentController.dataSource = dataSource
Fabric.with([Crashlytics.self])
return true
}