Flutter Stateful red screen (begginer) - flutter

Question

I'm starting out and trying to create a simple application layout with a side menuMy code:
import 'package:flutter/material.dart';
import 'package:flutter/foundation.dart';
import 'package:side_navigation/side_navigation.dart';
void main() {
runApp(const MainView());
}
class MainView extends StatefulWidget {
const MainView({Key? key}) : super(key: key);
#override
_MainViewState createState() => _MainViewState();
}
class _MainViewState extends State<MainView> {
List<Widget> views = const [
Center(
child: Text('Dashboard'),
),
Center(
child: Text('Account'),
),
Center(
child: Text('Settings'),
),
];
int selectedIndex = 0;
#override
Widget build(BuildContext context) {
return Scaffold(
body: Row(
children: [
SideNavigationBar(
selectedIndex: selectedIndex,
items: const [
SideNavigationBarItem(
icon: Icons.dashboard,
label: 'Dashboard',
),
SideNavigationBarItem(
icon: Icons.person,
label: 'Account',
),
SideNavigationBarItem(
icon: Icons.settings,
label: 'Settings',
),
],
onTap: (index) {
setState(() {
selectedIndex = index;
});
},
// Change the background color and disabled header/footer dividers
// Make use of standard() constructor for other themes
theme: SideNavigationBarTheme(
backgroundColor: Colors.grey,
togglerTheme: SideNavigationBarTogglerTheme.standard(),
itemTheme: SideNavigationBarItemTheme.standard(),
dividerTheme: SideNavigationBarDividerTheme.standard(),
),
),
Expanded(
child: views.elementAt(selectedIndex),
)
],
),
);
}
}
Source: https://pub.dev/packages/side_navigation#getting-started
Response: Red screen with no media query widget ancestor found....
I'm just starting to learn but I'm stuck on this
I tried different return types but it didn't help

Answer 1

Create a StatelessWidget which returns a MaterialApp, with your MainView widget as a children.
Like this:
class MyApp extends StatelessWidget{
const MyApp({Key? key}) : super(key: key);
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'My App',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: MainView(),
);
}
}
And then you use runApp(MyApp()), you always need a MaterialApp or CupertinoApp widget to run your application. They configure several stuff for you.

Answer 2

if MainView() is your root class then you are missing MaterialApp
create a StatelessWidget or StatefulWiget and return a Material App
example:
class MyApp extends StatelessWidget {
const MyApp({super.key});
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
// This is the theme of your application.
//
// Try running your application with "flutter run". You'll see the
// application has a blue toolbar. Then, without quitting the app, try
// changing the primarySwatch below to Colors.green and then invoke
// "hot reload" (press "r" in the console where you ran "flutter run",
// or simply save your changes to "hot reload" in a Flutter IDE).
// Notice that the counter didn't reset back to zero; the application
// is not restarted.
primarySwatch: Colors.blue,
),
home: MainView(),
);
}
}