TextFormField在验证时不显示错误

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text('TextFormField Validation')),
        body: MyForm(),
      ),
    );
  }
}

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

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

  String? _validateName(String? value) {
    if (value == null || value.isEmpty) {
      return 'Name is required';
    }
    return null;
  }

  @override
  Widget build(BuildContext context) {
    return Form(
      key: _formKey,
      child: Column(
        children: <Widget>[
          TextFormField(
            decoration: InputDecoration(labelText: 'Name'),
            validator: _validateName,
            onSaved: (String? value) {
              _name = value!;
            },
          ),
          ElevatedButton(
            onPressed: () {
              if (_formKey.currentState!.validate()) {
                _formKey.currentState!.save();
                // Handle form submission
              }
            },
            child: Text('Submit'),
          ),
        ],
      ),
    );
  }
}