我希望flutter应用程序在每次按下安卓系统的后退按钮时都能收到通知，而且，我还想禁用后退按钮，这是我在Kotlin方面的代码：

package com.example.ui

import io.flutter.embedding.android.FlutterActivity
import io.flutter.embedding.engine.FlutterEngine
import io.flutter.plugin.common.MethodChannel

class MainActivity: FlutterActivity() {

    override fun onBackPressed() {
        // send message to flutter that the system back button was pressed
        val flutterEngine = getFlutterEngine()
        if (flutterEngine != null) {
            MethodChannel(flutterEngine!!.dartExecutor.binaryMessenger, "backButtonChannel").invokeMethod("backButtonPressed", null)
        }
    }
}

这是我的flutter应用的主函数：

Future<void> backButtonPressed() async {
  // Code to run when the back button is pressed
  print('PRESSED!');
}

const platform = MethodChannel('com.example.ui/back_button');

void main() {
  WidgetsFlutterBinding.ensureInitialized();
  platform.setMethodCallHandler((call) async {
    if (call.method == "backButtonPressed") {
      return backButtonPressed();
    }
  });
  return runApp(MyApp());
}

不幸的是，它似乎没有效果，你能看出为什么吗？