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