如何在DropdownButtonFormField中设置边框圆角？使用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('DropdownButtonFormField'),
        ),
        body: Center(
          child: MyForm(),
        ),
      ),
    );
  }
}

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

class _MyFormState extends State<MyForm> {
  String _selectedItem;

  @override
  Widget build(BuildContext context) {
    return Container(
      padding: EdgeInsets.all(16.0),
      child: DropdownButtonFormField(
        decoration: InputDecoration(
          border: OutlineInputBorder(
            borderRadius: BorderRadius.circular(10.0),
          ),
        ),
        value: _selectedItem,
        items: [
          DropdownMenuItem(
            value: 'Option 1',
            child: Text('Option 1'),
          ),
          DropdownMenuItem(
            value: 'Option 2',
            child: Text('Option 2'),
          ),
          DropdownMenuItem(
            value: 'Option 3',
            child: Text('Option 3'),
          ),
        ],
        onChanged: (value) {
          setState(() {
            _selectedItem = value;
          });
        },
      ),
    );
  }
}