Null check operator used on a null Value ( List ) - flutter

Question

I'm trying to pass the data of a List for another screen but i don't know how to do that.
I've tried using constructors and using correctly null-safety but the app returned this error: Null check operator used on a null Value. And i'm a long time already trying to solve this problem but i don't know how. What's the benefits of null safety? Here's the code:
Code 1
import 'dart:convert';
import 'dart:io';
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:flutter_speed_dial/flutter_speed_dial.dart';
import 'package:mood3/telas/RecebeDados.dart';
import 'package:mood3/telas/listas.dart';
import 'package:path_provider/path_provider.dart';
import 'package:toast/toast.dart';
import 'Diary.dart';
class Escrita extends StatefulWidget {
List? lista = [];
Escrita({this.lista});
#override
State<Escrita> createState() => _EscritaState();
}
class _EscritaState extends State<Escrita> {
Future<File>_getFile()async{
final diretorio = await getApplicationDocumentsDirectory();
return File("${diretorio.path}/dados.json");
}
_salvar() async {
var arquivo = await _getFile();
Map<String,dynamic> tarefa = Map();
tarefa["titulo"] = "Ir ao mercado";
tarefa["realizada"] = false;
widget.lista!.add(tarefa);
String dados = json.encode(widget.lista);
arquivo.writeAsString(dados);
}
_ler() async {
try{
final arquivo = await _getFile();
return arquivo.readAsString();
}catch(e){
return null;
}
}
#override
void initState() {
super.initState();
_ler().then((dados){
setState(() {
widget.lista = json.decode(dados);
});
});
}
TextEditingController _controllerTarefa = TextEditingController();
#override
Widget build(BuildContext context) {
ToastContext().init(context);
return Scaffold(
body: Center(
child: SingleChildScrollView(
child:Column(
children: [
Padding(
padding: EdgeInsets.symmetric(horizontal: 8, vertical: 16),
child: TextField(
keyboardType: TextInputType.multiline,
controller: _controllerTarefa,
decoration: InputDecoration(
hintText: "Diary Title",
border: OutlineInputBorder(
borderRadius: BorderRadius.circular(8),
borderSide: BorderSide.none,
),
filled: true,
fillColor: Colors.blueGrey,
),
)),
Padding(
padding: EdgeInsets.symmetric(horizontal: 8, vertical: 16),
child: TextField(
keyboardType: TextInputType.multiline,
maxLines: null,
decoration: InputDecoration(
hintText: "Diary Text",
border: OutlineInputBorder(
borderRadius: BorderRadius.circular(8),
borderSide: BorderSide.none,
),
filled: true,
fillColor: Colors.blueGrey,
),
),),
],
),),
),
}
),
]
),
);
}
}
Code 2:
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:flutter_speed_dial/flutter_speed_dial.dart';
import 'package:mood3/telas/Account.dart';
import 'package:mood3/telas/Diary.dart';
import 'package:mood3/telas/Friends.dart';
import 'package:mood3/telas/Graph.dart';
import 'package:mood3/telas/Home.dart';
import 'package:mood3/telas/Music.dart';
import 'package:mood3/telas/PlayerDoVideo.dart';
import 'package:mood3/telas/Videos.dart';
import 'package:mood3/telas/animacao.dart';
import 'package:mood3/telas/escrita.dart';
import 'package:persistent_bottom_nav_bar_v2/persistent-tab-view.dart';
class RecebeDados extends StatefulWidget {
#override
State<RecebeDados> createState() => _RecebeDadosState();
}
class _RecebeDadosState extends State<RecebeDados> {
Escrita flista = Escrita();
late List? list = flista.lista;
#override
Widget build(BuildContext context) {
return Scaffold(
body: Column(
children: <Widget>[
Expanded(
child: ListView.builder(
itemCount: list!.length,
itemBuilder: (context, index){
return ListTile(
title: Text( list![index]['titulo'] ),
);
}
),
)
],
),
floatingActionButton: SpeedDial(
animatedIcon: AnimatedIcons.menu_close,
backgroundColor: Colors.black,
icon: CupertinoIcons.pencil,
overlayColor: Colors.black,
children: [
Text ("teste")
]
),
);
}
}

Answer 1

In this case you are creating "lista" as a List that could be null (List?), but also you initialize it with value: a empty list ( [] )
List? lista = [];
But when you call the constructor of Escrita, you are giving a value to the lista variable.
Widget myWidget = Escrita(); /// lista is null
lista is null, because you are not indicating a value in the constructor.
If you want a default value this is the correct way:
Escrita({this.lista = []});
You are receiving errors because you are using list!.length and list![index]['titulo'], and using the ! you are asecuring that the list is not null, but yes, its null! So thats the reason because you are receiving the error.

Answer 2

You can solve your problem in this way:
class Escrita extends StatefulWidget {
List lista;
Escrita({this.lista=[]});
#override
State<Escrita> createState() => _EscritaState();
}
Or you can do this:
class Escrita extends StatefulWidget {
List lista;
Escrita({required this.lista});
#override
State<Escrita> createState() => _EscritaState();
}
Or
class Escrita extends StatefulWidget {
List lista;
Escrita(this.lista);
#override
State<Escrita> createState() => _EscritaState();
}