在Flutter中，当我调用showSnackBar时，它会一直显示，直到我刷新页面。它不会自行消失

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('SnackBar Demo'),
        ),
        body: Center(
          child: RaisedButton(
            child: Text('Show SnackBar'),
            onPressed: () {
              final snackBar = SnackBar(
                content: Text('This is a SnackBar'),
                duration: Duration(seconds: 2), // 设置持续时间为2秒
              );
              Scaffold.of(context).showSnackBar(snackBar);
            },
          ),
        ),
      ),
    );
  }
}