使用下拉菜单时无法更改文本值- flutter

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('Dropdown Example'),
        ),
        body: Center(
          child: DropdownExample(),
        ),
      ),
    );
  }
}

class DropdownExample extends StatefulWidget {
  @override
  _DropdownExampleState createState() => _DropdownExampleState();
}

class _DropdownExampleState extends State<DropdownExample> {
  String _selectedValue = 'One';

  @override
  Widget build(BuildContext context) {
    return DropdownButton<String>(
      value: _selectedValue,
      onChanged: (String newValue) {
        setState(() {
          _selectedValue = newValue;
        });
      },
      items: <String>['One', 'Two', 'Free', 'Four']
          .map<DropdownMenuItem<String>>((String value) {
        return DropdownMenuItem<String>(
          value: value,
          child: Text(value),
        );
      }).toList(),
    );
  }
}