title: iOS WKWebView.scrollView delegate cause BAD_ACCESS
body: <p>My viewController has a UIView property which contains a WKWebView. And I set the WKWebView scrollView delegate to my viewController, which is a public function of my UIView subClass and called in my viewController. </p>
  
  <p>The issue is when I call <code>[viewController popViewControllerAnimated]</code>, it will crash on <code>[UIScrollView setDelegate:]</code>.
  I have fixed the issue by add <code>viewController.UIView.WKWebView.scrollView.delegate = nil;</code> in viewController's <strong>dealloc</strong>.</p>
  
  <p>But why? WKWebView's dealloc is after viewController's dealloc, I suppose viewController is set to nil and dealloc in WKWebView will update its delegate to nil then cause BAD_ACCESS? But why dealloc will inplicit call setDelegate???</p>