window.rootViewController nil while unwrapping

Question

I want to use Coordinator in my Swift project by using the file Navigator(), but I need to pass a UINavigationController inside to navigator.show().

The problem is that XCode shows this error:

Fatal Error: Unexpectedly found nil while unwrapping an Optional value in window!

AppDelegate

class AppDelegate: UIResponder, UIApplicationDelegate {

  var window: UIWindow!
  let navigator = Navigator()

  func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

    let feedNavigation = window!.rootViewController!  as! UINavigationController
    navigator.show(segue: .listBeer, sender: feedNavigation)
    
    return true
  }
}

How can I pass this window.rootViewController! as! UINavigationController as a non-Optional value?