[WebViewController] ios crash when page send undefined message via javascriptChannel

### Steps to reproduce

1. register javascript channel in webview controller
2. load page that will eventually send undefined message to this channel

### Expected results

onMessageReceived callback should receive instance of JavascriptMessage with 'undefined' message as a string.

### Actual results

App crashes with an exception

### Code sample

After 5 seconds  js will send deadly command to application.

<details open><summary>Code sample</summary>

```dart
import 'package:flutter/material.dart';
import 'package:webview_flutter/webview_flutter.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    WebViewController controller = WebViewController()
    ..setJavaScriptMode(JavaScriptMode.unrestricted)
    ..addJavaScriptChannel('Channel', onMessageReceived: (message) {
      print('Received message: ${message.toString()}');
    });

    NavigationDelegate navigationDelegate = NavigationDelegate(
      onPageFinished: (String url) {
        controller.runJavaScript("""
        var i=3;
        setInterval(() => {
          document.body.textContent = 'Crash in ' + i;
          --i;
        }, 1000);
 
        setTimeout(() => {
          window.Channel.postMessage(undefined);
        }, 5000);
        """);
      },
    );

     controller.setNavigationDelegate(navigationDelegate);
     controller.loadHtmlString('<html><body>Test</body></html>');

    return MaterialApp(
      home:  Scaffold(
          appBar: AppBar(title: const Text('60km/h crash test')),
          body: WebViewWidget(controller: controller),
    )
    );
  }
}
```

</details>


### Screenshots or Video

<details open>
<summary>Screenshots / Video demonstration</summary>

https://github.com/user-attachments/assets/19294ed7-fe88-45d2-97c4-b3f34f8bb26f

</details>


### Logs

<details open><summary>Logs</summary>

```console
webview_flutter_wkwebview/ProxyAPIRegistrar.swift:71: Fatal error: webview_flutter_wkwebview.PigeonError: Error returned from calling WKScriptMessageHandler.didReceiveScriptMessage: Optional("Null check operator used on a null value")
```

</details>


### Flutter Doctor output

<details open><summary>Doctor output</summary>

```console
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, 3.27.3, on macOS 15.1.1 24B91 darwin-arm64, locale pl-PL)
[✓] Android toolchain - develop for Android devices (Android SDK version 35.0.0-rc4)
[✓] Xcode - develop for iOS and macOS (Xcode 16.2)
[✓] Chrome - develop for the web
[✓] Android Studio (version 2024.1)
[✓] VS Code (version 1.96.4)
[✓] Connected device (5 available)            
[✓] Network resources

• No issues found!
```

</details>


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Uh oh!

[WebViewController] ios crash when page send undefined message via javascriptChannel #162352

Steps to reproduce

Expected results

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Uh oh!

[WebViewController] ios crash when page send undefined message via javascriptChannel #162352

Description

Steps to reproduce

Expected results

Actual results

Code sample

Screenshots or Video

Logs

Flutter Doctor output

Metadata

Metadata

Assignees

Labels

Type

Fields

Projects

Milestone

Relationships

Development

Issue actions