HttpException occure when i post a request to a server as below - flutter

Question

This method is to post an order to a server and it's in a Provider class :
Future<void> addOrder(OrderRequest orderRequest) async {
final prefs = await SharedPreferences.getInstance();
String accessToken = prefs.getString(Constants.prefsUserAccessTokenKey);
String url = Urls.addOrderUrl;
try {
var bodyParams = json.encode({
"Branch": {"Id": orderRequest.branchId},
"DeliveryAddress":
orderRequest.addressId == 0 ? {} : {"Id": orderRequest.addressId},
"InBranch": orderRequest.inBranch,
"TableNumber": orderRequest.tableNumber.toString(),
"OrderItems": orderRequest.items,
"PromoCode": orderRequest.promoCodeId == 0
? {}
: {"Id": orderRequest.promoCodeId}
});
print("Url: " + url);
print("Token: " + accessToken);
print("Params: " + bodyParams);
final response = await retry(
() => http
.post(url,
headers: {
"content-type": "application/json",
"Accept": "application/json",
"Authorization": "Bearer " + accessToken
},
body: json.encode(bodyParams))
.timeout(Duration(seconds: 5)),
retryIf: (e) => e is SocketException || e is TimeoutException);
final responseData = json.decode(response.body);
print(responseData);
if (response.statusCode == 200) {
} else if (response.statusCode == 401) {
throw AuthException("401", responseData['Message']);
} else {
throw HttpException(responseData['Message']);
}
} catch (error) {
print(error);
throw error;
}
}
and in my screen class i create a method to upload my data to the server which i use it when i press a button which handle the post request :
Future<void> _addOrder() async {
OrderRequest request = OrderRequest();
request.addressId = _selectedAddress.id;
request.branchId = int.parse(_selectedBranchId);
request.inBranch = _selectedAddress.id == 0;
request.items = _cartItemsList;
request.promoCodeId = _promoCodeId;
request.tableNumber = _tableNumber;
try {
setState(() {
_isLoading = true;
});
await Provider.of<OrderProvider>(context).addOrder(request);
Provider.of<CartProvider>(context).emptyCart();
_showDialog("Order Sent", "Your order is sent to restaurant.");
} on HttpException catch (error) {
_showDialog("Error adding order", error.message);
} on SocketException catch (_) {
_showDialog("Error adding order",
"Please check your internet connection and try again");
} on TimeoutException catch (_) {
_showDialog("Error adding order",
"Please check your internet connection and try again");
} on AuthException catch (_) {
_refreshToken();
} catch (error) {
print(error);
_showDialog("Error adding address", "Something went wrong");
}
}
but when i press a Order button t to send a post request to a server i got this error:
I/flutter (12421): {Message: Error:Object reference not set to an instance of an object.}
I/flutter (12421): HttpException: Error:Object reference not set to an instance of an object.

Answer 1

this is the model class that i use
class OrderRequest{
int branchId;
int addressId;
bool inBranch;
int promoCodeId;
int tableNumber;
List<CartItem> items;
OrderRequest(
{
this.branchId,
this.addressId,
this.inBranch,
this.promoCodeId,
this.tableNumber,
this.items});
}