Navigation Bar dissapearing when using rootViewController

I am trying to see if the user is already logged in, and if he is, to bring him to a navigation controller, which contains the mainViewController.

This is in my scene delegate

if UserDefaults.standard.bool(forKey: "isLoggedIn") {
  let homeViewController = UIStoryboard(name: "Main", bundle: nil).instantiateViewController(withIdentifier: "MainStoryBoard") as! MainViewController
  let navigationController = UINavigationController(rootViewController: homeViewController)
  window.rootViewController = navigationController            
}  

This is what it's supposed to look like

But, instead, it removes the navigation bar.