Try adding an initializer expression, or add a field initializer in this constructor, or mark it 'late' Non-nullable instance field ''_catalog' initia

Question

import 'package:flutter_one/models/catalog.dart';
import 'package:flutter/material.dart';

class CartModel {
  // singleton class
  static final cartModel = CartModel._internal();
  catalog._internal();
  factory CartModel() => cartModel;

  // catalog field
  CatalogModel _catalog;

  // Collection of IDs - store Ids of each item
  final List<int> _itemIds = [];

  // Get Catalog
  CatalogModel get catalog => _catalog;

  set catalog(CatalogModel newCatalog) {
    assert(newCatalog != null);
    _catalog = newCatalog;
  }

  // get items in the cart
  List<Item> get items => _itemIds.map((id) => _catalog.getById(id)).toList();

  // get total price
  num get totalPrice =>
      items.fold(0, (total, current) => total + current.price);

  // add item
  void add(Item item) {
    _itemIds.add(item.id);
  }

  // remove item
  void remove(Item item) {
    _itemIds.remove(item.id);
  }
}

Non-nullable instance field '_catalog' must be initialized. (Documentation) Try adding an initializer expression, or add a field initializer in this constructor, or mark it 'late'. The name of a constructor must match the name of the enclosing class.

Try adding an initializer expression, or add a field initializer in this constructor, or mark it 'late' Non-nullable instance field ''_catalog' initia

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