title: black screen with sceneDelegate when using storyboard
body: <p>First shows AuthViewController (aka root)
  then a black screen with tabBarController, but without item on it.</p>
  
  <p>What could be the problem?</p>
  
  <pre><code>func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
  
          guard let windowScene = (scene as? UIWindowScene) else { return }
          window = UIWindow(frame: UIScreen.main.bounds)
          window?.windowScene = windowScene
  
          if let user = Auth.auth().currentUser {
              FirestoreServices.shared.getUserData(user: user) { (result) in
                  switch result {
                  case .success(let muser):
                      let mainTabBar = MainTabBarViewController()
                      mainTabBar.currentUser = muser
                      mainTabBar.modalPresentationStyle = .fullScreen
                      self.window?.rootViewController = mainTabBar
                  case .failure(_):
                      self.window?.rootViewController = AuthViewController()
                  }
              }
          } else {
              self.window?.rootViewController = AuthViewController()
          }
          window?.makeKeyAndVisible()
      } 
  </code></pre>