我正在尝试flutter，我是这方面的新手，当我使用API SERVICE登录时遇到了问题（我使用Rest API https://reqres.in），登录说
_CastError（对空值使用的空检查运算符）
错误来自下面的源代码

Container(
              
              margin: const EdgeInsets.only(
                  left: 30, right: 30, top: 20, bottom: 20),
              padding: const EdgeInsets.symmetric(vertical: 10),
              width: double.infinity,
              child: ElevatedButton(
                onPressed: () {
                 if (Loginvalues.loginFormKey.currentState!.validate()) {
                    Loginvalues.userLogin(Loginvalues.loginEmailController, Loginvalues.loginPasswordController,);
                 }
                },
                style: ElevatedButton.styleFrom(
                    backgroundColor: const Color(0xff84c148),
                    elevation: 5,
                    padding: const EdgeInsets.all(15),
                    shape: RoundedRectangleBorder(
                        borderRadius: BorderRadius.circular(15))),
                child: const SizedBox(
                  height: 15,

                  child: Text('SIGN IN',
                      style: TextStyle(
                        color: Colors.white,
                        fontSize: 15,
                        fontWeight: FontWeight.bold,
                      )),
                ),
              )),

在线：

onPressed: () {
   if (Loginvalues.loginFormKey.currentState!.validate()) { 
    Loginvalues.userLogin(Loginvalues.loginEmailController,  
    Loginvalues.loginPasswordController,);

这是我的登录控制器dart，其中loginFormkey被调用：

// ignore_for_file: prefer_const_constructors

import 'dart:convert';
import 'package:get/get.dart';
import 'package:crud_sample/screens/home_page.dart';
import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;

class LoginController with ChangeNotifier {
  TextEditingController loginEmailController = TextEditingController();
  TextEditingController loginPasswordController = TextEditingController();
  GlobalKey<FormState> loginFormKey = GlobalKey();

  userLogin(
    TextEditingController loginEmailController,
    TextEditingController loginPasswordController,
  ) async {
    try{
         var headers = {"Content-Type": "application/json"};
         var body = {
            'email' : loginEmailController.text.trim(),
            'password' : loginPasswordController.text.trim()
    };
    
       http.Response response = await http.post(
        Uri.parse('https://reqres.in/api/login'),
        headers: headers,
        body: jsonEncode(body),
        );
        
        if(response.statusCode == 200){
          Map json = jsonDecode(response.body.toString());
          print(json['token']);

          if(json['token'] != ''){
            Get.to(() => HomePage());
          }else{
            print("Invalid Details!");
          }
        }   
    }catch(e){
      print("Error $e");
    }
  }
}

我的依赖关系
库比蒂诺_icons：^1.0.2 http：^0.13.6获取：^4.6.5提供程序：6.0.5级
我做错了吗？
Fix _CastError（用于空值的空检查运算符）