Firebase realtime database onvalue event don't work

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Dart

Flutter

I have firebase realtime database. When I manually change data on the Firebase console, vscode stops responding and throws the following error.

Error: TypeError: Instance of '_Exception': type '_Exception' is not a subtype of type 'JavaScriptObject'

Main.dart:

import 'package:firebase_core/firebase_core.dart';
import 'package:firebase_database/firebase_database.dart';
import 'package:flutter/material.dart';
import 'package:realtimedbtest/firebase_options.dart';


String result = "";

void main()  async { 
WidgetsFlutterBinding.ensureInitialized(); 
await Firebase.initializeApp(
options: DefaultFirebaseOptions.currentPlatform,   
 );

DatabaseReference ref = FirebaseDatabase.instance.ref('widget/image/name'); 
ref.onValue.listen((DatabaseEvent event)     { 
result =   event.snapshot.value.toString();
});

runApp(const MainApp());
}

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

@override
  Widget build(BuildContext context) {
   return  MaterialApp(
    home: Scaffold(
      body: Center(
       child: Text("result: $result"),
      ),
     ),
   );
  }
}

What I have tried:

I put async

DatabaseReference ref = FirebaseDatabase.instance.ref('widget/image/name'); 
ref.onValue.listen((DatabaseEvent event) async { //<-- I put async here.
result = event.snapshot.value.toString();
});