Flutter:传递给下一个屏幕的变量变为空

// 第一个屏幕
class FirstScreen extends StatelessWidget {
  final String variable;

  FirstScreen({required this.variable});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('First Screen'),
      ),
      body: Center(
        child: ElevatedButton(
          child: Text('Go to Second Screen'),
          onPressed: () {
            Navigator.push(
              context,
              MaterialPageRoute(
                builder: (context) => SecondScreen(variable: variable),
              ),
            );
          },
        ),
      ),
    );
  }
}

// 第二个屏幕
class SecondScreen extends StatelessWidget {
  final String variable;

  SecondScreen({required this.variable});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Second Screen'),
      ),
      body: Center(
        child: Text(variable),
      ),
    );
  }
}