Created attachment 312599 [details] An Xcode Swift project demonstrating the issue. Overview: The WKNavigationAction.sourceFrame property is not annotated as nullable, causing it to crash when accessed from Swift. This bug shows up when making a navigation request that does not originate from the page itself using one of the WKWebView load methods. I've also filed this bug with Apple under rdar://32686815. Steps to Reproduce: This bug can be reproduced from both Objective-C and Swift but is more critical in Swift, as access of the property causes crashing: 1. Create a WKWebView and set its navigationDelegate to an object implementing the WKNavigationDelegate protocol method `webView(WKWebView, decidePolicyFor: WKNavigationAction, decisionHandler: (WKNavigationActionPolicy) -> Void)` insert `print(navigationAction.sourceFrame)` in its body. 2. Create a new navigation load request for the WKWebView using `webView.loadHTMLString(String)` 3. Run the project. Actual Results: The sourceFrame property is bridged as non-nullable, and upon access the application will crash. Expected Results: `nil` should be printed. Build Date & Hardware: iOS 10, 9 & OSX 10.11 >.
<rdar://problem/32728039>