Error: No Firebase App '[DEFAULT]' has been created - call firebase.initializeApp() in react native code example

Example 1: [core/no-app] No Firebase App '[DEFAULT]' has been created - call Firebase.initializeApp() flutter

import 'package:firebase_core/firebase_core.dart';

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  runApp(MyApp());
}

Example 2: [core/no-app] No Firebase App '[DEFAULT]' has been created - call Firebase.initializeApp() "flutter"

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  var fsconnect = FirebaseFirestore.instance;

  myget() async {
    var d = await fsconnect.collection("students").get();
    // print(d.docs[0].data());

    for (var i in d.docs) {
      print(i.data());
    }
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        home: Scaffold(
      appBar: AppBar(
        title: Text('Firebase Firestore App'),
      ),
      body: Column(
        children: [
          RaisedButton(
            child: Text('send data'),
            onPressed: () {
              fsconnect.collection("students").add({
                'name': 'sarah',
                'title': 'xyz',
                'email': '[email protected]',
              });
              print("send ..");
            },
          ),
          RaisedButton(
            child: Text('get data'),
            onPressed: () {
              myget();
              print("get data ...");
            },
          )
        ],
      ),
    ));
  }
}