title: Self.revealViewController() returns nil
body: <p>I am currently trying to get the pan gesture to reveal the controller. As of right now, my open button works, but when I add the last line, my program crashes because revealViewController is nil. Link to pod: <a href="https://github.com/John-Lluch/SWRevealViewController" rel="nofollow noreferrer">https://github.com/John-Lluch/SWRevealViewController</a>. </p>
  
  <pre><code>    import Foundation
      import UIKit
      class MainViewController : UIViewController{
      @IBOutlet weak var mainButton: UIView!
      @IBOutlet weak var Open: UIBarButtonItem!
      override func viewDidLoad() {
          super.viewDidLoad()
  
      }
      override func viewDidAppear(_ animated: Bool) {
          super.viewDidAppear(animated);
          mainButton.makeCircle();
          Open.target = self.revealViewController()
          Open.action = #selector(SWRevealViewController.revealToggle(_:));
          self.view.addGestureRecognizer(self.revealViewController().panGestureRecognizer());
      }
  }
  </code></pre>