How to fix Method "Contains key" called on null flutter

Question

I am a beginner at Flutter and I've been working on an e-commerce app. I used the Provider package to add a product to the cart but I get the following error: My flutter doctor is also just fine. I am having this problem only when I try to add an item to the cart. Any help would be appreciated.

══╡ EXCEPTION CAUGHT BY GESTURE ╞═══════════════════════════════════════════════════════════════════
The following NoSuchMethodError was thrown while handling a gesture:
The method 'containsKey' was called on null.
Receiver: null
Tried calling: containsKey("p1")
When the exception was thrown, this was the stack:
#0      Object.noSuchMethod  (dart:core-patch/object_patch.dart:51:5)
#1      Cart.addItem 
package:shop_stop/…/components/cart.dart:33
#2      _ProductItemState.build.<anonymous closure> 
package:shop_stop/widgets/product_item.dart:80

My cart provider:

class CartItem {
  final String id;
  final String title;
  final int quantity;
  final double price;

  CartItem({
    @required this.id,
    @required this.title,
    @required this.quantity,
    @required this.price,
  });
}

class Cart with ChangeNotifier {
  Map<String, CartItem> _items;

  Map<String, CartItem> get items {
    return {..._items};
  }

  int get itemCount {
    return _items.length;
  }

  void addItem(
    String productId,
    double price,
    String title,
  ) {
    if (_items.containsKey(productId)) {
      // change quantity...
      _items.update(
        productId,
        (existingCartItem) => CartItem(
          id: existingCartItem.id,
          title: existingCartItem.title,
          price: existingCartItem.price,
          quantity: existingCartItem.quantity + 1,
        ),
      );
    } else {
      _items.putIfAbsent(
        productId,
        () => CartItem(
          id: DateTime.now().toString(),
          title: title,
          price: price,
          quantity: 1,
        ),
      );
    }
    notifyListeners();
  }
}

My product Item:

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import 'package:shop_stop/screens/cart/components/cart.dart';

import '../views/product_details.dart';
import '../models/product.dart';

class ProductItem extends StatefulWidget {
  @override
  _ProductItemState createState() => _ProductItemState();
}

class _ProductItemState extends State<ProductItem> {
  @override
  Widget build(BuildContext context) {
    final products = Provider.of<Product>(context, listen: false);
    final cart = Provider.of<Cart>(context, listen: false);
    return Container(
      child: GestureDetector(
        onTap: () {
          Navigator.of(context)
              .pushNamed(ProductDetails.routeName, arguments: products.id);
        },
        child: Card(
          color: Colors.white,
          elevation: 6,
          shape:
              RoundedRectangleBorder(borderRadius: BorderRadius.circular(10)),
          child: Column(
            crossAxisAlignment: CrossAxisAlignment.start,
            children: <Widget>[
              //product image
              Image(image: AssetImage('assets/laptop.jpg')),
              Padding(
                padding: EdgeInsets.symmetric(vertical: 0, horizontal: 10),
                //item name and fav button
                child: Flex(
                  direction: Axis.horizontal,
                  children: <Widget>[
                    Expanded(
                        child: Text(
                      products.name,
                      style: Theme.of(context)
                          .textTheme
                          .headline1
                          .copyWith(fontSize: 16, color: Colors.black),
                    )),
                    //add-to-fav button
                    Consumer<Product>(
                      builder: (ctx, product, child) => IconButton(
                          icon: Icon(
                            products.isFav
                                ? Icons.favorite
                                : Icons.favorite_border,
                            color: Colors.red,
                          ),
                          onPressed: () {
                            products.toggleFav();
                          }),
                    )
                  ],
                ),
              ),
              // price and buy button
              Padding(
                padding: EdgeInsets.symmetric(horizontal: 10, vertical: 0),
                child: Flex(
                  direction: Axis.horizontal,
                  children: <Widget>[
                    Expanded(
                        child: Text(
                      "\$ " + products.price.toString(),
                      style: Theme.of(context).textTheme.headline1.copyWith(
                          color: Theme.of(context).primaryColor, fontSize: 15),
                    )),
                    SizedBox(
                      width: 60,
                      child: RaisedButton(
                        onPressed: () {
                          cart.addItem(
                              products.id, products.price, products.name);
                        },
                        color: Colors.orange,
                        child: Text(
                          "Buy",
                          style: TextStyle(fontFamily: 'Prompt', fontSize: 14),
                        ),
                      ),
                    )
                  ],
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}

Answer 1

Change Map<String, CartItem> _items; to Map<String, CartItem> _items = {};