The method 'data' was called on null. Receiver: null Tried calling: data() - flutter

Question

When I try to put the value added to Firebase into the Droppdown button, I get the following error
The method 'data' was called on null.Receiver: nullTried calling: data ()
I think the cause of the error is that I can't call the value from Todo.dart to post_page.dart.
Using debag, todolist is null in post_page.dart.
I would appreciate it if you could answer.
post_page.dart
import 'package:cloud_firestore/cloud_firestore.dart';
import "package:flutter/material.dart";
import 'package:habit_boddy/component/post/task_add_button.dart';
import 'package:habit_boddy/utils/constants.dart';
import 'package:habit_boddy/view/common/components/drop_text.dart';
import 'package:habit_boddy/view/common/components/post_caption_part.dart';
import 'package:habit_boddy/view/post/page/Todo.dart';
import 'package:habit_boddy/view/post/page/detail_page.dart';
import 'package:habit_boddy/view/post/page/picture_page.dart';
import 'package:habit_boddy/view/post/page/task_setting.dart';
import 'package:habit_boddy/view_models/post_view_model.dart';
import 'package:habit_boddy/view_models/todo_view_model.dart';
import 'package:provider/provider.dart';
import 'confirm_dialog.dart';
class PostPage extends StatefulWidget {
#override
_PostPageState createState() => _PostPageState();
}
class _PostPageState extends State<PostPage> {
final _captionController = TextEditingController();
#override
void initState() {
_captionController.addListener(_onCaptionUpdated);
super.initState();
}
#override
void dispose() {
_captionController.dispose();
super.dispose();
}
#override
Widget build(BuildContext context) {
return ChangeNotifierProvider<ToDoViewModel>(
create:(_) => ToDoViewModel()..getRealtime(),
child: Scaffold(
appBar: AppBar(
backgroundColor: Colors.amber,
actions: [
ElevatedButton(
child: Text('Post'),
style: ElevatedButton.styleFrom(
primary: Colors.orange,
onPrimary: Colors.white,
),
onPressed: () => showConfirmedDialog(
context: context,
title: "Post",
content: "May I post?",
onConfirmed: (isConfirmed) {
if (isConfirmed) {
_post(context);
}
}))
],
),
body: Consumer<ToDoViewModel>(builder: (context, model, child) {
final todoList = model.todoList;
return DropdownButton<Todo>(
items: todoList.map((todo) =>
DropdownMenuItem<Todo>(
value: Todo(title: todo.title),
child: Text(todo.title),
)
).toList(),
);
}),
floatingActionButton:
Consumer<ToDoViewModel>(builder: (context, model, child) {
return FloatingActionButton(
onPressed: () async {
await Navigator.push(
context,
MaterialPageRoute(builder: (context) => TaskSetting()),
);
},
child: Icon(Icons.add),
);
}),
),
);
}
_onCaptionUpdated() {
final viewModel = Provider.of<PostViewModel>(context, listen: false);
viewModel.caption = _captionController.text;
print("caption: ${viewModel.caption}");
}
void _post(BuildContext context) {
final postViewModel = Provider.of<PostViewModel>(context, listen: false);
}
}
Widget build(BuildContext context) {
final postViewModel = Provider.of<PostViewModel>(context);
return Container(
child: Padding(
padding: const EdgeInsets.all(40.0),
child: SingleChildScrollView(
child: Column(
children: [
Row(
children: [
Expanded(
flex: 3,
child: Container(
child: Padding(
padding: const EdgeInsets.only(left: 10.0),
child: DropText(),
),
height: 230),
),
Expanded(
flex: 1,
child: Padding(
padding: const EdgeInsets.only(bottom: 155.0),
child: TaskAdd(),
)),
],
),
Container(
child: Padding(
padding: const EdgeInsets.only(top: 20.0),
child: DetailPost(),
),
height: 270),
Row(
children: [
PicturePage(),
postViewModel.imageFile == null
? Container()
: Container(
height: 60,
width: 60,
child: SingleChildScrollView(
child: PostCaptionPart(
from: PostCaptionOpenMode.FROM_POST,
),
),
)
],
),
],
),
),
),
);
}
Todo.dart
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';
import 'package:habit_boddy/view_models/todo_view_model.dart';
class Todo {
final ToDoViewModel model;
String title = "";
DateTime createdAt;
Todo({DocumentSnapshot doc, this.title, this.model, this.createdAt}){
this.title = doc.data()["title"];
final Timestamp timestamp = doc.data()['createdAt'];
this.createdAt = timestamp.toDate();
}
}

Answer 1

I believe this line is causing the error.
DropdownMenuItem<Todo>(
value: Todo(title: todo.title),
You're calling doc.data() in the Todo constructor without passing the doc variable, hence calling .data() on a null variable. You should instead get the title first, then pass it to the constructor when instantiating the Todo object.
If you really need to pass the doc variable, say if you need it for something else, make it required in the constructor and assert that it is not null.
Todo({#required DocumentSnapshot doc, this.title, this.model, this.createdAt}) : assert(doc != null){
...
}
You should look into Dart null safety as well, which would help to avoid this type of issue.