如何将未来的字符串设置为TextFormField的'initial value‘

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  final String futureString = "未来的字符串";

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('设置未来的字符串'),
        ),
        body: Center(
          child: TextFormField(
            initialValue: futureString,
            decoration: InputDecoration(
              labelText: '未来的字符串',
            ),
          ),
        ),
      ),
    );
  }
}