title: Why WKWebView becomes nil?
body: <p>I use Swift4.0 and Xcode9.0. I try to show a website using <code>WKWebView</code>. It had no problem until yesterday, but today an error occurred.</p>
  
  <pre><code>import UIKit
  import WebKit
  
  class WKWebViewViewController: UIViewController, WKUIDelegate, WKNavigationDelegate {
  
      @IBOutlet weak var webView: WKWebView!
  
      var startUrl = "https://www.apple.com"
      var requestUrl: String?
  
      override func loadView() {
          webView.uiDelegate = self
          webView.navigationDelegate = self
  
      }
  
      override func viewDidLoad() {
          super.viewDidLoad()
  
          webView.allowsBackForwardNavigationGestures = true
  
          if let url = URL(string: startUrl) {
              let request = URLRequest(url: url)
              webView.load(request)
  
          }
  
      }
  
  }
  </code></pre>
  
  <p>The error occurs at <code>webView.uiDelegate = self</code>. Here's the error log:</p>
  
  <pre><code>fatal error: unexpectedly found nil while unwrapping an Optional value
  </code></pre>
  
  <p>For debug, I added <code>print("webView:\(webView)")</code> under <code>loadView()</code>, and I found webView itself is <code>nil</code>.</p>
  
  <p>I tried to add <code>?</code> to all <code>webView</code>, but only black screen appeared on the app.
  Why <code>WkWebView</code> becomes nil? Is it some required setting outside my code?</p>