Flutter Http getting a response but passing null to then when the method is call - flutter

Question

I actually get the response from my API, using this method.
static Future<String> saveData(String url, Vehicle vehicle) async {
dio.Dio d = dio.Dio();
Map<String, dynamic> headers = {};
headers['Cookie'] = "JSESSIONID=" + "fff";// SessionUtils().getSession().toString();
dio.Options options = dio.Options(
headers: headers,
contentType: 'application/json',
responseType: dio.ResponseType.json,
);
final dio.Response response = await d.post(url, options: options, data: jsonEncode(vehicle));
print(response.data);
dynamic item;
String result = "";
if (response.statusCode == HttpStatus.ok) {
item = response.data; //json.decode(response.data);
print("****************");
print(item);
print("****************");
print(item["id"]);
if(item["success"]){
result = item["id"]; // **result is correct here**
print("OK");
}
print(item["success"]); // Data is printed out correctly
}
print(result);
return result;
}
But when I call this method using the code below I cannot get the returned data in the then.
VehicleHttpService.saveData(Constant.POST_VEHICLE_URL, widget.vehicle).then((value){
}).then((String? value){
print("#########");
print(value); //**I get null here. why**
print("#########");
}).whenComplete((){
Navigator.of(context).pop();
});
I was expecting the result returned by saveData. Please help me check what is wrong with my code.

Answer 1

Let's check your executing code by parts:
VehicleHttpService.saveData(Constant.POST_VEHICLE_URL, widget.vehicle).then((value){
}).then((String? value){
print("#########");
print(value); //**I get null here. why**
print("#########");
}).whenComplete((){
Navigator.of(context).pop();
});
The first step is executing the API request and return a response
VehicleHttpService.saveData(Constant.POST_VEHICLE_URL, widget.vehicle)
The second step is receives the response, converts it in null and return to the Future chain
then((value){})
The third step always will receive the null
then((String? value){
print("#########");
print(value); //**I get null here. why**
print("#########");
}).whenComplete((){
Navigator.of(context).pop();
})
Remove then in the middle and you will receive the response and can able to handle it