1. 처음 뷰를 띄울 viewController를 변수로 받는다.
let mainViewController = MainViewController()
let navigationController = UINavigationController(rootViewController: mainViewController)
window?.rootViewController = navigationController
window?.makeKeyAndVisible()
override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(animated)
self.view.backgroundColor = .systemBackground
}