如何以编程方式清除Flutter DropdownButton？

class MyDropdown extends StatefulWidget {
  @override
  _MyDropdownState createState() => _MyDropdownState();
}

class _MyDropdownState extends State<MyDropdown> {
  GlobalKey<_MyDropdownState> _dropdownKey = GlobalKey();
  String _selectedItem;

  @override
  Widget build(BuildContext context) {
    // ... 
  }
}

String _selectedItem;

void _onSelectChanged(String newValue) {
  setState(() {
    _selectedItem = newValue;
  });
}

@override
Widget build(BuildContext context) {
  return DropdownButton<String>(
    key: _dropdownKey,
    value: _selectedItem,
    onChanged: _onSelectChanged,
    items: [
      DropdownMenuItem(value: 'Item 1', child: Text('Item 1')),
      DropdownMenuItem(value: 'Item 2', child: Text('Item 2')),
      DropdownMenuItem(value: 'Item 3', child: Text('Item 3')),
    ],
  );
}

void clearDropdown() {
  setState(() {
    _selectedItem = null;
  });
}

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('DropdownButton Example')),
        body: MyDropdown(),
      ),
    );
  }
}

class MyDropdown extends StatefulWidget {
  @override
  _MyDropdownState createState() => _MyDropdownState();
}

class _MyDropdownState extends State<MyDropdown> {
  GlobalKey<_MyDropdownState> _dropdownKey = GlobalKey();
  String _selectedItem;

  void _onSelectChanged(String newValue) {
    setState(() {
      _selectedItem = newValue;
    });
  }

  void clearDropdown() {
    setState(() {
      _selectedItem = null;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        DropdownButton<String>(
          key: _dropdownKey,
          value: _selectedItem,
          onChanged: _onSelectChanged,
          items: [
            DropdownMenuItem(value: 'Item 1', child: Text('Item 1')),
            DropdownMenuItem(value: 'Item 2', child: Text('Item 2')),
            DropdownMenuItem(value: 'Item 3', child: Text('Item 3')),
          ],
        ),
        ElevatedButton(
          onPressed: clearDropdown,
          child: Text('Clear Dropdown'),
        ),
      ],
    );
  }
}