WKWebView is not displaying content in custom UIView Swift

Question

In my storyboard I have something like a individual navigation bar (three buttons in a row) and below this navbar there should be a WKWebView.

So I create a UIView called webViewSize and implement the WKWebView.

 var webView: WKWebView!
 @IBOutlet weak var webViewSize: UIView!

 override func viewDidLoad() {
    super.viewDidLoad()

    webView = WKWebView()
    webView.navigationDelegate = self
    self.webViewSize = self.webView

    dispatch_async(dispatch_get_main_queue()) {
        if self.url.characters.count > 0 {
            if let url = NSURL(string: self.url) {
                self.webView.loadRequest(NSURLRequest(URL: url))
                self.webView.allowsBackForwardNavigationGestures = true
            }
        }
    } 
  }

After a few seconds the WK delegate didFinishNavigation is called, but nothing is shown. When I change self.webViewSize = self.webView to self.view = self.webView everything works fine.

I searched a lot to handle this. I worked with subviews, repeat the line self.webViewSize = self.webView in didFinishNavigation, but nothing work.

What's my mistake?

PS: I know it's Swift 2, but it's an old(er) project. Atm I have to handle this. It will be convert in time.