如何在flutter中显示TextForm域边界内的标签和错误信息？

import 'package:flutter/material.dart';

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

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

  @override
  void dispose() {
    _textController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Form(
      key: _formKey,
      child: TextFormField(
        controller: _textController,
        decoration: InputDecoration(
          labelText: '标签',
          border: OutlineInputBorder(),
        ),
        validator: (value) {
          if (value.isEmpty) {
            return '错误信息';
          }
          return null;
        },
      ),
    );
  }
}