在Flutter中使用TextField（）构建UI时，我在state中定义了控制器，并尝试使用TextField（）中定义的emailController和passwordController，但它显示“无效常量值"。我尝试解决该问题，但没有成功。以下是login_screen.dart的代码

import 'package:flutter/material.dart';
import 'package:rider_app/routes/routing_constants.dart';

class LoginScreen extends StatefulWidget {
  const LoginScreen({Key? key}) : super(key: key);

  @override
  State<LoginScreen> createState() => _LoginScreenState();
}

class _LoginScreenState extends State<LoginScreen> {
  final TextEditingController emailController = TextEditingController();
  final TextEditingController passwordController = TextEditingController();
  final _validate = false;
  
  // @override
  // void dispose() {
  //   emailController.dispose();
  //   passwordController.dispose();
  //   super.dispose();
  // }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      body: SingleChildScrollView(
        child: Column(
          children: [
            const SizedBox(
              height: 40.0,
            ),
            const Center(
                child: Image(
              image: AssetImage("lib/assets/images/logo.png"),
              width: 200.0,
              height: 200.0,
              alignment: Alignment.center,
            )),
            const SizedBox(
              height: 2.0,
            ),
            const Text(
              "Login as a Rider",
              textAlign: TextAlign.center,
              style: TextStyle(fontSize: 25.0, fontFamily: "Brand Bold"),
            ),
            Padding(
              padding:
                  const EdgeInsets.symmetric(horizontal: 30.0, vertical: 10.0),
              child: Column(
                children: [
                  const SizedBox(
                    height: 10.0,
                  ),
                  const TextField(
                    
                    keyboardType: TextInputType.emailAddress,
                    decoration: InputDecoration(
                      labelText: "Email",
                      labelStyle: TextStyle(
                        fontSize: 15.0,
                      ),
                      hintText: "Email address",
                      hintStyle: TextStyle(fontSize: 12.0, color: Colors.grey),
                      // focusedBorder: OutlineInputBorder(
                      //   borderSide:
                      //       BorderSide(width: 1.0, color: Colors.blueAccent),
                      // ),
                      // enabledBorder: OutlineInputBorder(
                      //   borderSide: BorderSide(width: 1.0),
                      // ),
                    ),
                    style: TextStyle(fontSize: 15.0),
                    controller: emailController,
                    
                  ),
                  const SizedBox(
                    height: 10.0,
                  ),
                  const TextField(
                    controller: passwordController,
                    keyboardType: TextInputType.visiblePassword,
                    decoration: InputDecoration(
                      labelText: "Password",
                      labelStyle: TextStyle(fontSize: 15.0),
                      hintText: "Your password",
                      hintStyle: TextStyle(fontSize: 12.0, color: Colors.grey),
                      // focusedBorder: OutlineInputBorder(
                      //   borderSide: BorderSide(width: 1.0, color: Colors.blueAccent),
                    ),
                    // enabledBorder: OutlineInputBorder(
                    //   borderSide: BorderSide(width: 1.0)
                    // )
                    // ),
                    obscureText: true,
                    style: TextStyle(fontSize: 15.0),
                  ),
                  const SizedBox(
                    height: 45.0,
                  ),
                  ElevatedButton(
                    onPressed: () {
                      debugPrint("Logged In");
                      Navigator.pushNamed(context, homeScreen);
                    },
                    child: const Padding(
                      padding: EdgeInsets.symmetric(
                          horizontal: 23, vertical: 11),
                      child: Text(
                        'Login',
                        style: TextStyle(
                            fontSize: 18,
                            fontFamily: "Brand Bold",
                            fontWeight: FontWeight.w500),
                      ),
                    ),
                  ),
                ],
              ),
            ),
            FlatButton(
                onPressed: () {
                  Navigator.pushNamed(context, signUpScreen);
                },
                child: Row(
                  mainAxisAlignment: MainAxisAlignment.center,
                  children: const [
                    Text(
                      "Don't have Account?",
                      style: TextStyle(
                          fontWeight: FontWeight.w400,
                          fontFamily: "Brand-Regular"),
                    ),
                    Text(
                      " Register Here.",
                      style: TextStyle(
                          fontFamily: "Brand-Regular",
                          fontWeight: FontWeight.w600),
                    )
                  ],
                ))
          ],
        ),
      ),
    );
  }
}

控制器定义是正确的，但当我试图将控制器分配给TextField()控制器时，抛出了错误。

有什么想法？？让我知道，即使是建议也很感激。谢谢！
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