Error "The method [] was called on null" in Flutter while parsing Json

I was creating a Flutter application which parses a simple JSON, and it works well but gives an error in Debug Console in VS Code,

My simple JSON file:

With Lightest Code snippet :

import 'package:flutter/material.dart'; import 'dart:convert'; void main() => runApp(new MyApp()); class MyApp extends StatelessWidget { @override Widget build(BuildContext context) { return new MaterialApp(title: "J2", home: new Homepage()); } } class Homepage extends StatefulWidget { @override State createState() => new HomepageState(); } class HomepageState extends State<Homepage> { @override Widget build(BuildContext context) { return new Scaffold( appBar: new AppBar(title: new Text("J2")), body: new Container( child: new FutureBuilder( future: DefaultAssetBundle.of(context) .loadString('assets/data.json'), builder: (context, snapshot) { Map<String, dynamic> mydata = json.decode(snapshot.data.toString()); return new Container(child: new Text(mydata['city'])); }))); } } 

And it works well but with this error in the debug console, how can I overcome it? I am a beginner.