how to use function of Viewcontroller in AppDelegate in swift

I get framework from external. so I want to use native code in flutter. If user click button, it shows new page.

error message

Swift Compiler Error (Xcode): Editor placeholder in source file AppDelegate.swift:18:48

No-usage (Xcode): Function is unused AppDelegate.swift:18:10

Uncategorized (Xcode): Command EmitSwiftModule failed with a nonzero exit code

If I change ViewController.openNewPage(<#T##self: ViewController##ViewController#>) to result(100). it works very well.

Flutter code

ElevatedButton(onPressed: _openNewpage, child: Text("button"))
Future _openNewpage() async {
   await MethodChannel("io").invokeMethod("openNewPage");
}

AppDelegate.swift

import UIKit
import Flutter

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
  ) -> Bool {
      let controller : FlutterViewController = window?.rootViewController as! FlutterViewController
      let openChannel = FlutterMethodChannel(name:"io", binaryMessenger: controller.binaryMessenger)
      openChannel.setMethodCallHandler({
          (call:FlutterMethodCall, result:@escaping FlutterResult)->Void in
          guard call.method == "openNewPage" else{
              result(FlutterMethodNotImplemented)
              return
          }
          ViewController.openNewPage(<#T##self: ViewController##ViewController#>)
      })
      GeneratedPluginRegistrant.register(with: self)
          return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }
}

ViewController.swift

class ViewController: FlutterViewController {
    var openScreen: FinpongPlusScreen!

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
    }
    
    
    func openNewPage(_ sender:Any) {
        finpongPlus = FinpongPlusScreen()                      
        finpongPlus.finpongPlusDelegate = self                  
        finpongPlus.modalPresentationStyle = .fullScreen      
        finpongPlus.mode = FinpongPlusScreen.Mode.SAMPLE       
        finpongPlus.entry = "0"                                 
        finpongPlus.log = true                                 

        present(finpongPlus, animated: true, completion: nil)
    }