push ViewController without storyboard

Question

I try to push a view controller from the main ViewController like this (Swift 4):

@objc func childAction(sender: UIButton!) {
    print("Child button tapped")
    let vc = childDetailViewController()
    self.navigationController?.pushViewController(vc, animated: true)
}

The text is printed, but the viewController is not pushed. What did I missed?

Answer 1

Inside AppDelegate's didFinishLaunchingWithOptions do

let fir = FirstVC()
self.window?.rootViewController = UINavigationController(rootViewController: fir)

Then this

self.navigationController?.pushViewController(vc, animated: true)

should work