Error method post ( HTTP Request ) Flutter

Just check out the example that i have created from your example.

import 'package:flutter/material.dart';

import 'package:sample_list/post_login.dart';

void main() {
  runApp(new MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(home: Home());
  }
}

class Home extends StatefulWidget {
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  String url = "http://13.250.59.224:56321/api/users/login";
  @override
  void initState() {
    super.initState();
    callAPI();
  }

  callAPI() {
    Post post = Post();
    createPost(post, url).then((response) {
      if (response.statusCode == 200) {
        print(response.body);
      } else {
        print(response.statusCode);
      }
    }).catchError((error) {
      print('error : $error');
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(),
        body: FutureBuilder<Post>(
            future: getPost(url, "OAISJFAOISFJASF"),
            builder: (context, snapshot) {
              //callAPI(); // It will get called every time for every build
              if (snapshot.connectionState == ConnectionState.done) {
                if (snapshot.hasError) {
                  return Text("Error");
                }
                return Text('UserID JSON : ${snapshot.data.userId}');
              } else
                return CircularProgressIndicator();
            }));
  }
}

//This is the post-class

import 'dart:async';
import 'dart:io';
import 'dart:convert';

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

// To parse this JSON data, do
//
//     final post = postFromJson(jsonString);


Post postFromJson(String str) => Post.fromJson(json.decode(str));

String postToJson(Post data) => json.encode(data.toJson());

//  Nạp và phân tích cú pháp dữ liệu JSON
Future<Post> getPost(String url, String firebase_token) async {
  Map<String, String> headers = {"Content-type": "application/json"};
  String json =
      '{"grant_type":"firebase_token","firebase_token":"$firebase_token"}';
  final response = await http.post(url, headers: headers, body: json);
  print('string 1');
  print(response.body);
  return postFromJson(response.body);
}

Future<http.Response> createPost(Post post, String url) async {
  final response = await http.post('$url',
      headers: {
        HttpHeaders.contentTypeHeader: 'application/json',
        HttpHeaders.authorizationHeader: ''
      },
      body: postToJson(post));
  return response;
}

class Post {
  Post({
    this.userId,
    this.accessToken,
    this.tokenType,
    this.expiresUtc,
    this.issuedUtc,
    this.refreshToken,
    this.roles,
  });

  String userId;
  String accessToken;
  String tokenType;
  DateTime expiresUtc;
  DateTime issuedUtc;
  String refreshToken;
  List<String> roles;

  factory Post.fromJson(Map<String, dynamic> json) => Post(
        userId: json["user_id"],
        accessToken: json["access_token"],
        tokenType: json["token_type"],
        expiresUtc: DateTime.parse(json["expires_utc"]),
        issuedUtc: DateTime.parse(json["issued_utc"]),
        refreshToken: json["refresh_token"],
        roles: List<String>.from(json["roles"].map((x) => x)),
      );

  Map<String, dynamic> toJson() => {
        "user_id": userId,
        "access_token": accessToken,
        "token_type": tokenType,
        "expires_utc": expiresUtc.toIso8601String(),
        "issued_utc": issuedUtc.toIso8601String(),
        "refresh_token": refreshToken,
        "roles": List<dynamic>.from(roles.map((x) => x)),
      };
}

Just make you changes accordingly.

Let me know if it works.