Unhandled Exception: type 'Null' is not a subtype of type 'Map<String, dynamic>' in type cast

Question

import 'dart:convert';

import 'package:flutter/foundation.dart';
import 'package:http/http.dart' as http;

import './cart.dart';

class OrderItem {
 final String id;
  final double amount;
  final List<CartItem> products;
  final DateTime dateTime;

  OrderItem({
    required this.id,
    required this.amount,
    required this.products,
    required this.dateTime,
  });
}

class Orders with ChangeNotifier {
  List<OrderItem> _orders = [];

  List<OrderItem> get orders {
    return [..._orders];
  }

The errored block( when I do if check to null, vscode says The operand can't be null, so the condition is always false. also, I get an error saying Unhandled Exception: type 'Null' is not a subtype of type 'Map<String, dynamic>' in typecast.

  Future<void> fetchAndSetOrders() async {
    final url =
        Uri.https('shopapp-63-default-rtdb.firebaseio.com', '/orders.json');

    final response = await http.get(url);
    final List<OrderItem> loadedOrders = [];
    final extractedData = json.decode(response.body) as Map<String, dynamic>;
    if (extractedData == null) {
      return;
    }

    extractedData.forEach((orderId, orderData) {
      loadedOrders.add(
        OrderItem(
          id: orderId,
          amount: orderData['amount'],
          dateTime: DateTime.parse(orderData['dateTime']),
          products: (orderData['products'] as List<dynamic>)
              .map((item) => CartItem(
                    id: item['id'],
                    title: item['title'],
                    quantity: item['quantity'],
                    price: item['price'],
                  ))
              .toList(),
        ),
      );
    });
    _orders = loadedOrders;
    notifyListeners();
  }

Answer 1

I think you are using Flutter 2.0+ which has nullsafety. Thus you can not check for a null to a variable which you do not define nullable explicitly.

Try defining

    final response = await http.get(url);
    final extractedData = json.de...

as

    final response? = await http.get(url);
    final extractedData? = json....

to slove The operand can't be null, so the condition is always false Then you can check null using ?? operator or with the if condition you are already using.

I don't know why null is not a subtype of type Map<String, dynamic> is occurring. Because your http.get(url) should not return null by any means.

Answer 2

try this method instead.

{
    final response = await http.get(url);
    final _data = json.decode(response.body);
    if (_data != null) {
        final extractedData = _data as Map<String, dynamic>;
    }