Flutter 在 Firebase 数据中没有记录时向控制台显示错误

我正在从Firebase获取订单数据，工作正常，但如果没有数据，它会在控制台中显示错误

我添加了一个if语句，如果数据不可用，即使它不能解决问题

这是我的代码，用于获取订单数据，并在Firebase数据中没有产品的情况下处理产品列表

那么，如果没有产品，现在该如何解决呢

Future<void> fetchandsetorders() async {
    final url = Uri.parse(
        myelin);
    final response = await http.get(url);
    final List<OrderItem> loadedorders = [];
    final extradeddata = json.decode(response.body) as Map<String, dynamic>;
    
//I added this statement but still not solving
if(extradeddata==null)
      return;
    extradeddata.forEach((orderid, orderdata) {
      loadedorders.add(
        OrderItem(
          id: orderid,
          products: (orderdata['products'] as List<dynamic>).map((item) {
            return CartItem(
                id: item['id'],
                title: item['title'],
                quantity: item['qty'],
                price: item['price']);
          }).toList(),
          amount: orderdata['amount'],
          date: DateTime.parse(orderdata['date']),
        ),

      );
    });

    _orders=loadedorders;
    notifyListeners();
  }

显示错误是

E/flutter (32144): [ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: type 'Null' is not a subtype of type 'Map<String, dynamic>' in type cast
E/flutter (32144): #0      Orders.fetchandsetorders (package:finalshopapp/orders.dart:31:53)
E/flutter (32144): <asynchronous suspension>