[webview_flutter_wkwebview] Implementation of WebViewWidget with WebKit (flutter#6342)

Author: bparrishMines

packages/webview_flutter/webview_flutter_wkwebview/lib/src/v4/src/webkit_navigation_delegate.dart

@@ -36,7 +36,7 @@ class WebKitProxy {
       String keyPath,
       NSObject object,
       Map<NSKeyValueChangeKey, Object?> change,
-    )
+    )?
         observeValue,
   }) createWebView;