Flutter cannot call result in native ios code

Question

Good day, maybe someone can help me here. I have a SDK built into Flutter that I call via the ChannelMethod. There I enter a callback whose result I want to give back to flutter. The callback works but for some unknown reason the result function is not triggered.

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

    let controller : FlutterViewController = window?.rootViewController as! FlutterViewController
    let testChannel = FlutterMethodChannel(name: "test",
                                              binaryMessenger: controller.binaryMessenger)
    testChannel.setMethodCallHandler({
        [weak self] (call: FlutterMethodCall, result: @escaping FlutterResult) -> Void in
      // This method is invoked on the UI thread.
      guard call.method == "startProcess" else {
          result(FlutterMethodNotImplemented)
        return
      }
        
    let args = call.arguments as! Dictionary<String, Any>

        let mainVC = ViewController()
        mainVC.callBack = { (resultValue: String) in
            print("Callback called:")
            print(resultValue)
            // <-------- this does not work -------->
            result(resultValue)
            result("test")
          };
        mainVC.caseId = args["caseId"] as! String
        let navigationController = UINavigationController(rootViewController: mainVC)
    self?.window.rootViewController = navigationController
    self?.window.makeKeyAndVisible()
    })
    
    GeneratedPluginRegistrant.register(with: self)
    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }
}