如何在颤动中显示3秒的CircularProgressIndicator

dependencies:
  flutter:
    sdk: flutter

import 'dart:async';
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('Circular Progress Indicator Demo'),
        ),
        body: Center(
          child: MyCircularProgressIndicator(),
        ),
      ),
    );
  }
}

class MyCircularProgressIndicator extends StatefulWidget {
  @override
  _MyCircularProgressIndicatorState createState() =>
      _MyCircularProgressIndicatorState();
}

class _MyCircularProgressIndicatorState
    extends State<MyCircularProgressIndicator> {
  bool _isLoading = true;

  @override
  void initState() {
    super.initState();
    Timer(Duration(seconds: 3), () {
      setState(() {
        _isLoading = false;
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    return _isLoading ? CircularProgressIndicator() : Container();
  }
}