nullsafety 버전에 따른 firestore 연동 오류 해결

1. firestore 버전 업 (pubspec.yaml)

# Add the dependency for the Firebase Core Flutter SDK
firebase_core: ^1.2.1
cloud_firestore: ^2.2.1

2. main.dart 에서 
Firebase.initializeApp();

import 'package:flutter/material.dart';
import 'package:flutter_netflix_clone/screen/home_screen.dart';
import 'package:flutter_netflix_clone/screen/more_screen.dart';
import 'package:flutter_netflix_clone/widget/bottom_bar.dart';
import 'package:firebase_core/firebase_core.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  late TabController controller;


  @override
  Widget build(BuildContext context) {
    Firebase.initializeApp();

3. home screnn에서 

class _HomeScreenState extends State<HomeScreen> {
  FirebaseFirestore firestore = FirebaseFirestore.instance;

4. model_movie.dart 에서

class Movie {
  final String title;
  final String keyword;
  final String poster;
  final bool like;
  final DocumentReference reference;

  Movie.fromMap(Map<String, dynamic> map, {required this.reference})
      : title = map['title'],
        keyword = map['keyword'],
        poster = map['poster'],
        like = map['like'];

  Movie.fromSnapShot(DocumentSnapshot snapshot)
      : this.fromMap(snapshot.data() as Map<String, dynamic>, reference: snapshot.reference);

5. build.gradle 설정 

════════ Exception caught by widgets library ═══════════════════════════════════
The following FirebaseException was thrown building KeyedSubtree-[<0>]:
[core/no-app] No Firebase App '[DEFAULT]' has been created - call Firebase.initializeApp()

The relevant error-causing widget was
TabBarView
lib/main.dart:32
When the exception was thrown, this was the stack
#0      MethodChannelFirebase.app
package:firebase_core_platform_interface/…/method_channel/method_channel_firebase.dart:122
#1      Firebase.app
package:firebase_core/src/firebase.dart:54
#2      FirebaseFirestore.instance
package:cloud_firestore/src/firestore.dart:27
#3      new _HomeScreenState
package:untitled/screen/home_screen.dart:13
#4      HomeScreen.createState
package:untitled/screen/home_screen.dart:9
...
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