我的表单键在flutter中无法验证，即使我在表单中将其用作键并输入合适的值之后也是如此

import 'package:flutter/material.dart';

class MyForm extends StatefulWidget {
  @override
  _MyFormState createState() => _MyFormState();
}

class _MyFormState extends State<MyForm> {
  final _formKey = GlobalKey<FormState>();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Form Validation'),
      ),
      body: Form(
        key: _formKey,
        child: Column(
          children: [
            TextFormField(
              validator: (value) {
                if (value.isEmpty) {
                  return 'Please enter some text';
                }
                return null;
              },
            ),
            ElevatedButton(
              onPressed: () {
                if (_formKey.currentState.validate()) {
                  // 验证通过，执行提交操作
                }
              },
              child: Text('Submit'),
            ),
          ],
        ),
      ),
    );
  }
}

void main() {
  runApp(MaterialApp(
    home: MyForm(),
  ));
}