UIViewController, Dismiss and Present @@ in Swift3 - Xcode 8.0 iOS 10

기존에 열려있는 firstViewController 를 Dismiss 하고 secondViewController를 Present 하기 위해

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self.dismiss(animated: true, completion: {
  let vc = self.storyboard?.instantiateViewController(withIdentifier: "secondViewController")
  self.present(vc!, animated: true, completion: nil)
})
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아래 오류가 뜬다.
whose view is not in the window hierarchy!

이때는 뷰계층구조상 rootViewController를 사용해야한다.
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self.dismiss(animated: true, completion: {
   let vc = self.storyboard?.instantiateViewController(withIdentifier: "secondViewController")
   let appDelegate = UIApplication.shared.delegate as! AppDelegate
   appDelegate.window?.rootViewController!.present(vc, animated: true, completion: nil) 

})

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이렇게 하면 기존의 firstViewController는 닫히고 secondViewController 가 열린다