Flutter:如何正确地从JSON中读取字段?

请检查以下颤振代码

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
import 'dart:convert' as convert;
import 'dart:io';


class UserService with ChangeNotifier {
  NavLinks _navLinks = NavLinks();
  late User _user;

  Future<int> saveUser(User user, String authToken) async {
    print(convert.json.encode(user.toJson()));
    int responsetag = 0;
    try {
      await http.post(Uri.parse(_navLinks.saveUser()),
          body: convert.json.encode(user.toJson()),
          headers: {
            "Accept": "application/json",
            "content-type": "application/json",
            HttpHeaders.authorizationHeader: "Bearer $authToken"
          }).then((http.Response response) {
        final int statusCode = response.statusCode;

        print("RESPONSE: " + response.body);
        print("STATUS CODE: " + statusCode.toString());

        if (statusCode < 200 || statusCode > 400) {
          throw new Exception("Error while fetching data");
        } else {
          Map<String, dynamic> data = convert.jsonDecode(response.body);

          responsetag = int.parse(data["insertId"]);
        }
      });

      return responsetag;
    } catch (error) {
      throw (error);
    }
  }
}

在这里我需要提取 insertId ,将其转换为int并传递。但我总是以错误告终 [ERROR:flutter/lib/ui/ui_dart_state.cc(199)] Unhandled Exception: Exception: type 'int' is not a subtype of type 'String' .

这里出了什么问题?