无法在Flutter应用程序中隐藏进度对话框

import 'package:flutter/material.dart';

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  bool _showProgressDialog = false;

  void _showDialog() {
    setState(() {
      _showProgressDialog = true;
    });
  }

  void _hideDialog() {
    setState(() {
      _showProgressDialog = false;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Hide Progress Dialog Example'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            RaisedButton(
              child: Text('Show Progress Dialog'),
              onPressed: _showDialog,
            ),
            RaisedButton(
              child: Text('Hide Progress Dialog'),
              onPressed: _hideDialog,
            ),
          ],
        ),
      ),
      // 显示进度对话框
      floatingActionButton: _showProgressDialog
          ? FloatingActionButton(
              onPressed: null,
              child: CircularProgressIndicator(),
            )
          : null,
    );
  }
}

void main() {
  runApp(MaterialApp(
    home: MyWidget(),
  ));
}