Reproducibility 100% on iOS 15.4 and iOS 16.6 Zero crash on iOS 18.6 Xcode 26.1 Steps to Reproduce Xcode 26.1 → New iOS App Replace ViewController.swift with the 20-line code below Run on real device • iPhone XR iOS 15.4 • iPhone 13 iOS 16.6 Tap the link → breakpoint in decidePolicyFor lldb → po navigationAction.sourceFrame Actual Result (lldb) po navigationAction.sourceFrame nil Swift declaration lies: public var sourceFrame: WKFrameInfo { get } // non-optional → Instant EXC_BREAKPOINT libswiftFoundation.dylib`URLRequest._unconditionallyBridgeFromObjectiveC Objective-C tells the truth: po [(WKNavigationAction *)navigationAction fixedSourceFrame] nil iOS 18.6 → same code prints a valid WKFrameInfo, no crash. Expected sourceFrame must be declared WKFrameInfo? in Swift or at least documented “can be nil on iOS 15–16”. Impact Every WKWebView app that touches sourceFrame on iOS 15.4 & 16.6 ships with a latent crash. Production Workaround @implementation WKNavigationAction (Safe) (WKFrameInfo *)fixedSourceFrame { return self.sourceFrame ? self.sourceFrame : nil; } @end Minimal Test (copy-paste) import UIKit import WebKit class ViewController: UIViewController, WKNavigationDelegate { lazy var web = WKWebView(frame: view.bounds) override func viewDidLoad() { super.viewDidLoad() web.navigationDelegate = self view.addSubview(web) web.load(URLRequest(url: URL(string: "https://www.apple.com")!)) } func webView(_ webView: WKWebView, decidePolicyFor navigationAction: WKNavigationAction, preferences: WKWebpagePreferences, decisionHandler: @escaping (WKNavigationActionPolicy, WKWebpagePreferences)->Void) { print(navigationAction.sourceFrame) // ← crashes on 15.4 & 16.6 decisionHandler(.allow, preferences) } }