Sending data with nested JSON - flutter

Question

I have a two textfields; (body and user), which i want to parse their data to my json(nested). but the exception is being thrown instead
The snippet below is my JSON representation
{
"id": 27,
"user": {
"username": "admin"
},
"body": "Saturday morning",
"updated": "2022-07-30T06:48:53.009515Z",
"created": "2022-07-30T06:48:53.009515Z"
},
below is how i am sending the data
Future<Note> createNote(String body, String username) async {
final response = await http.post(
noteUrl,
headers: <String, String>{
'Content-Type': 'application/json; charset=UTF-8',
},
body: jsonEncode(<String, dynamic>{
'body': body,
'user': username,
}),
);
if (response.statusCode == 201) {
return Note.fromJson(jsonDecode(response.body));
} else {
throw Exception('Failed to create note');
}
}
this is where i am calling the function to send the data
ElevatedButton(
onPressed: () {
createNote(_bodyController.text, _userController.text);
Navigator.popAndPushNamed(context, '/');
},
child: Text('submit'),
)
Below snippet is my model class(flutter)
import 'dart:convert';
List<Note> noteFromJson(String str) =>
List<Note>.from(json.decode(str).map((x) => Note.fromJson(x)));
String noteToJson(List<Note> data) =>
json.encode(List<dynamic>.from(data.map((x) => x.toJson())));
class Note {
Note({
this.id,
required this.body,
this.updated,
this.created,
this.user,
});
int? id;
String body;
DateTime? updated;
DateTime? created;
User? user;
factory Note.fromJson(Map<String, dynamic> json) => Note(
id: json["id"],
body: json["body"] as String,
updated: DateTime.parse(json["updated"]),
created: DateTime.parse(json["created"]),
user: User.fromJson(json["user"]),
);
Map<String, dynamic> toJson() => {
"id": id,
"body": body,
"updated": updated?.toIso8601String(),
"created": created?.toIso8601String(),
"user": user?.toJson()
};
}
List<User> userFromJson(String str) =>
List<User>.from(json.decode(str).map((x) => User.fromJson(x)));
String userToJson(List<User> data) =>
json.encode(List<dynamic>.from(data.map((x) => x.toJson())));
class User {
User({
this.id,
required this.username,
this.password,
this.email,
});
String? id;
String username;
String? password;
String? email;
factory User.fromJson(Map<String, dynamic> json) => User(
id: json["id"],
username: json["username"],
password: json["password"],
email: json["email"],
);
Map<String, dynamic> toJson() => {
"id": id,
"username": username,
"password": password,
"email": email,
};
}