Fatal error: use of unimplemented initializer in custom navigationcontroller

UINavigationController's implementation of init(rootViewController:) probably calls self.init(nibName:bundle:) which you haven't implemented so it throws the error.

You should override init(nibName:bundle) in addition to the initializers you already override. init(nibName:bundle:) is a designated initializer while init(rootViewController:) is a convenience initializer.

While using custom navigation controller, we need to use override initproperty of NavigationController as-

class CustomNavigationController: UINavigationController {

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)

    }
    override init(rootViewController: UIViewController) {
        super.init(rootViewController: rootViewController)
    }
    override init(nibName nibNameOrNil: String?, bundle nibBundleOrNil: Bundle?) {
        super.init(nibName: nibNameOrNil, bundle: nibBundleOrNil)
    }

}

& in Appdelegate class use -

import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        self.window = UIWindow(frame: UIScreen.main.bounds)
        let vc = ViewController(nibName: "ViewController", bundle: nil)
        let navi =  CustomNavigationController(rootViewController: vc)
        window?.backgroundColor = .white
        window?.rootViewController = navi
        window?.makeKeyAndVisible()
        return true
    }
}

As per Apple Document - To simplify the relationships between designated and convenience initializers, Swift applies the following three rules for delegation calls between initializers:

Rule 1 A designated initializer must call a designated initializer from its immediate superclass.

Rule 2 A convenience initializer must call another initializer from the same class.

Rule 3 A convenience initializer must ultimately call a designated initializer.

A simple way to remember this is:-