Preserve undefined and null output

packages/webview_flutter/webview_flutter_wkwebview/example/integration_test/webview_flutter_test.dart

@@ -278,7 +278,7 @@ Future<void> main() async {
     await expectLater(channelCompleter.future, completion('hello'));
   });
 
-  testWidgets('JavaScriptChannel can receive undefined',
+  testWidgets('JavaScriptChannel can receive undefined and null',
       (WidgetTester tester) async {
     final Completer<void> pageFinished = Completer<void>();
     final PlatformWebViewController controller = PlatformWebViewController(
@@ -291,12 +291,12 @@ Future<void> main() async {
     unawaited(delegate.setOnPageFinished((_) => pageFinished.complete()));
     unawaited(controller.setPlatformNavigationDelegate(delegate));
 
-    final Completer<String> channelCompleter = Completer<String>();
+    final List<String> messages = <String>[];
     await controller.addJavaScriptChannel(
       JavaScriptChannelParams(
         name: 'Channel',
         onMessageReceived: (JavaScriptMessage message) {
-          channelCompleter.complete(message.message);
+          messages.add(message.message);
         },
       ),
     );
@@ -316,7 +316,10 @@ Future<void> main() async {
     await pageFinished.future;
 
     await controller.runJavaScript('Channel.postMessage(undefined);');
-    await expectLater(channelCompleter.future, completion(''));
+    await controller.runJavaScript('Channel.postMessage(null);');
+
+    await tester.pumpAndSettle();
+    expect(messages, <String>['undefined', 'null']);
   });
 
   testWidgets('resize webview', (WidgetTester tester) async {

packages/webview_flutter/webview_flutter_wkwebview/lib/src/webkit_webview_controller.dart

@@ -443,6 +443,7 @@ class WebKitWebViewController extends PlatformWebViewController {
         webKitParams.name,
       ),
     ]);
+    await _injectPostMessageOverride(webKitParams.name);
   }
 
   @override
@@ -768,6 +769,47 @@ window.addEventListener("error", function(e) {
     await controller.addUserScript(overrideScript);
   }
 
+  Future<void> _injectPostMessageOverride(String channelName) async {
+    final WKUserScript overrideScript =
+        _webKitParams.webKitProxy.newWKUserScript(
+      source: '''
+var _flutter_webview_plugin_overrides = _flutter_webview_plugin_overrides || {};
+
+_flutter_webview_plugin_overrides.originalPostMessageFunctions = _flutter_webview_plugin_overrides.originalPostMessageFunctions || {};
+
+(function() {
+  if (window.webkit && window.webkit.messageHandlers) {
+    const handlerName = "$channelName";
+    const handler = window.webkit.messageHandlers[handlerName];
+
+    if (handler && 
+        typeof handler.postMessage === 'function' &&
+        !_flutter_webview_plugin_overrides.originalPostMessageFunctions[handlerName]) {
+
+      _flutter_webview_plugin_overrides.originalPostMessageFunctions[handlerName] = handler.postMessage;
+
+      handler.postMessage = function(data) {
+        if (data === undefined) {
+          return _flutter_webview_plugin_overrides.originalPostMessageFunctions[handlerName].call(this, "undefined");
+        }
+        if (data === null) {
+          return _flutter_webview_plugin_overrides.originalPostMessageFunctions[handlerName].call(this, "null");
+        }
+        return _flutter_webview_plugin_overrides.originalPostMessageFunctions[handlerName].call(this, data);
+      };
+    }
+  }
+})();
+      ''',
+      injectionTime: UserScriptInjectionTime.atDocumentStart,
+      isForMainFrameOnly: false,
+    );
+
+    final WKUserContentController controller =
+        await _webView.configuration.getUserContentController();
+    await controller.addUserScript(overrideScript);
+  }
+
   // WKWebView does not support removing a single user script, so all user
   // scripts and all message handlers are removed instead. And the JavaScript
   // channels that shouldn't be removed are re-registered. Note that this

packages/webview_flutter/webview_flutter_wkwebview/test/webkit_webview_controller_test.dart

@@ -863,7 +863,7 @@ void main() {
       final WKUserScript userScript =
           verify(mockUserContentController.addUserScript(captureAny))
               .captured
-              .single as WKUserScript;
+              .toList().first as WKUserScript;
       expect(userScript.source, 'window.name = webkit.messageHandlers.name;');
       expect(
         userScript.injectionTime,
@@ -1008,7 +1008,7 @@ void main() {
       final WKUserScript userScript =
           verify(mockUserContentController.addUserScript(captureAny))
               .captured
-              .single as WKUserScript;
+              .toList().first as WKUserScript;
       expect(userScript.source, 'window.name2 = webkit.messageHandlers.name2;');
       expect(
         userScript.injectionTime,