title: Pushing Viewcontroller through navigation controller , Showing Black screen
body: <p>I being searching this issue in stack overflow but couldn't get an exact answer to the issue, i being stuck in it for long time.
  Mine issue is i'm trying to push a TestViewController through navigation controller. when i click the button the TestViewController is being load with navigation bar and the UIScreen is in black colour.</p>
  
  <p>Code of <strong>TestViewController</strong></p>
  
  <pre><code>import UIKit
  class TestViewController: UIViewController {
  
      override func viewDidLoad() {
          super.viewDidLoad()
  
          // Do any additional setup after loading the view.
      }   
  
      override func didReceiveMemoryWarning() {
          super.didReceiveMemoryWarning()
          // Dispose of any resources that can be recreated.
      }
      override func viewWillAppear(animated: Bool) {
          super.viewWillAppear(true)
          navigationItem.title = "Test page"
      }
  }
  </code></pre>
  
  <p>Code of Button </p>
  
  <pre><code>@IBAction func secondButtonClicked(sender: AnyObject) {
      buttonPressedNumber = "Two Clicked"
      buttonTextColor = UIColor.magentaColor()
        let a = TestViewController()
        let b:UIViewController = a as UIViewController
        navigationController?.pushViewController(b, animated: false)
  }
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/udG5k.png" rel="noreferrer"><img src="https://i.stack.imgur.com/udG5k.png" alt="enter image description here"></a></p>