颤振：初始屏幕中不会放置设备方向

Question

我在启动屏幕上使用了自定义设备方向（已运行），并尝试使用dispose（）将其恢复为正常状态。但这不起作用。

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这是我的代码-

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import 'dart:async';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:boimarket/widgets/exitpopup.dart';
import 'package:boimarket/homepage.dart';

class SplashScreen extends StatefulWidget {
  @override
  _SplashScreenState createState() => _SplashScreenState();
}

class _SplashScreenState extends State<SplashScreen> {
  @override
  void initState() {
    super.initState();
    SystemChrome.setPreferredOrientations([
      DeviceOrientation.portraitDown,
      DeviceOrientation.portraitUp,
    ]);

    Timer(
        Duration(seconds: 3),
        () => Navigator.push(context,
            MaterialPageRoute(builder: (context) => ExitPopUp(Homepage()))));
  }

  @override
  dispose() {
    SystemChrome.setPreferredOrientations([
      DeviceOrientation.portraitDown,
      DeviceOrientation.portraitUp,
      DeviceOrientation.landscapeRight,
      DeviceOrientation.landscapeLeft,
    ]);
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SingleChildScrollView(
        child: Container(
          child: Text('Hello')
    );
  }
}

问题出在哪里？以及我该如何解决？