Flutter module used in iOS native app: Unhandled Exception with shared_preferences initialization - PlatformException(channel-error)

Question

I have been searching on Stack Overflow for a long time, but I haven't found a suitable solution. Most of the answers either don't work or don't provide a clear answer, such as this post: SharedPreference giving "Unable to establish connection on channel." when used inside flutter_background_service. or https://stackoverflow.com/q/72880037/4964666

I am using a Flutter module in an iOS Swift app, and I am encountering the following error when initializing shared_preferences:

Unhandled Exception: PlatformException(channel-error, Unable to establish connection on channel., null, null)

This is my flutter doctor summary:

Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, 3.7.8, on macOS 12.6 21G115 darwin-x64, locale
    zh-Hant-TW)
[✓] Android toolchain - develop for Android devices (Android SDK version 31.0.0)
[✓] Xcode - develop for iOS and macOS (Xcode 14.1)
[✓] Chrome - develop for the web
[✓] Android Studio (version 2022.1)
[✓] VS Code (version 1.76.1)
[✓] Connected device (5 available)
[✓] HTTP Host Availability

• No issues found!

Here is how I am referencing the Flutter module in my Swift code:

In AppDelegate.swift:

lazy var flutterEngine = FlutterEngine(name: "myModuleEngine")

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    flutterEngine.run()
    GeneratedPluginRegistrant.register(with: self.flutterEngine)
    return true
}

In ViewController.swift:

let flutterEngine = (UIApplication.shared.delegate as! AppDelegate).flutterEngine
var api: NativeDataSaveToFlutterAPI!

override func viewDidLoad() {
    super.viewDidLoad()
    api = NativeDataSaveToFlutterAPI(binaryMessenger: flutterEngine.binaryMessenger)
}

@IBAction func goToFlutterPage(_ sender: Any) {
    self.flutterEngine.run(withEntrypoint: nil, initialRoute: "/")
    api = NativeDataSaveToFlutterAPI(binaryMessenger: flutterEngine.binaryMessenger)
    api.syncDataToFlutter(user: user) {
        let flutterViewController = FlutterViewController(project: nil, initialRoute: "/", nibName: nil, bundle: nil)
        self.present(flutterViewController, animated: true, completion: nil)
    }
}

Flutter main.dart

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  

  // The following code prints "before Shared init" but throws the error
  // "Unhandled Exception: PlatformException(channel-error, Unable to establish connection on channel., null, null)"
  // before it can print "after Shared init". However, the FlutterViewController is still being opened.
  print('before Shared init');
  await Shared.init();
  print('after Shared init');

  runApp(const MyExampleApp());
}

Flutter Shared class

class Shared {
  static late SharedPreferences _prefs;

  static init() async {
    _prefs = await SharedPreferences.getInstance();
  }

  static void setInt(String key, int value) async {
    await _prefs.setInt(key, value);
  }

  static int getInt(String key) {
    return _prefs.getInt(key) ?? 0;
  }
}

How can I resolve this error? I have confirmed that my Flutter module is functioning properly. Thank you!