Non-nullable instance field '_selectedSize' must be initialized - flutter

Question

I have been working with my Store App, but this null safety is getting me pissed now. I have created a class but it gives me this error with later doesn't allow my app to work correctly
this is the the product.dart file:
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';
import 'package:loja_virtual_nnananene/models/item_size.dart';
class Product extends ChangeNotifier {
Product.fromDocument(DocumentSnapshot document) {
id = document.documentID;
name = document['name'];
description = document['description'];
images = List<String>.from(document.data['images'] as List<dynamic>);
// ingore_for_file: Warning: Operand of null-aware operation
sizes = (document.data['sizes'] as List<dynamic> ?? [])
.map((s) => ItemSize.fromMap(s as Map<String, dynamic>))
.toList();
}
String id = "";
String name = "";
String description = "";
List<String> images = [];
List<ItemSize> sizes = [];
ItemSize _selectedSize;
ItemSize get selectedSize => _selectedSize;
set selectedSize(ItemSize value) {
_selectedSize = value;
notifyListeners();
}
}
I'm receiving an error at the Product.from...
This is the error:
Non-nullable instance field '_selectedSize' must be initialized.
Try adding an initializer expression, or add a field initializer in this constructor, or mark it 'late'.
This is my ItemSize class:
class ItemSize {
ItemSize.fromMap(Map<String, dynamic> map) {
name = map['name'] as String;
price = map['price'] as num;
stock = map['stock'] as int;
}
String name = "";
num price = 0;
int stock = 0;
bool get hasStock => stock > 0;
#override
String toString() {
return 'ItemSize{name: $name, price: $price, stock: $stock}';
}
}
Calling in main widget:
class SizeWidget extends StatelessWidget {
const SizeWidget(this.size);
final ItemSize size;
#override
Widget build(BuildContext context) {
final product = context.watch<Product>();
final selected = size == product.selectedSize;
Color color;
if (!size.hasStock)
color = Colors.red.withAlpha(50);
else if (selected)
color = ColorSelect.cprice;

Answer 1

the code tries to get the selected item the first time, and of course it will be null. The alternative I found was...
in ItemSize class, i create a construtor simple with all null -> ItemSize();
class ItemSize {
ItemSize.fromMap(Map<String, dynamic> map) {
name = map['name'] as String;
price = map['price'] as num;
stock = map['stock'] as int;
}
ItemSize();
String? name;
num? price;
int? stock;
bool get hasStock => stock! > 0;
#override
String toString() {
return 'ItemSize{name: $name, price: $price, stock: $stock}';
}
}
in the Product class do the get this way.
ItemSize get selectedSize {
if (_selectedSize != null)
return _selectedSize!;
else
return ItemSize();
}