Flutter: Strange animation combining bottomSheet and bottomNavigationBar and calling setState - flutter

Question

Im having a Scaffold with a bottomSheet and a bottomNavigationBar in a stateful widget.
Within the bottomSheet, i plan to add buttons, calling setState and triggering a rebuild of the local widget tree.
The problem is, that there is a strange animation happening when calling setState or when rebuilding the widgets. The bottom sheet appears to come from the bottom on top of the bottomNavigationbar.
Is there a way to fix that, such that this animation does not happen, when rebuilding or calling setState?
Please see the code below. To observe the behaviour, run the code and click on the green bottomSheet:
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
/// This Widget is the main application widget.
class MyApp extends StatelessWidget {
static const String _title = 'Flutter Code Sample';
#override
Widget build(BuildContext context) {
return MaterialApp(
title: _title,
home: MyStatefulWidget(),
);
}
}
class MyStatefulWidget extends StatefulWidget {
MyStatefulWidget({Key key}) : super(key: key);
#override
_MyStatefulWidgetState createState() => _MyStatefulWidgetState();
}
class _MyStatefulWidgetState extends State<MyStatefulWidget> {
int _selectedIndex = 0;
static const TextStyle optionStyle =
TextStyle(fontSize: 30, fontWeight: FontWeight.bold);
static const List<Widget> _widgetOptions = <Widget>[
Text(
'Index 0: Home',
style: optionStyle,
),
Text(
'Index 1: Business',
style: optionStyle,
),
Text(
'Index 2: School',
style: optionStyle,
),
];
void _onItemTapped(int index) {
setState(() {
_selectedIndex = index;
});
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: const Text('BottomNavigationBar Sample'),
),
body: Center(
child: _widgetOptions.elementAt(_selectedIndex),
),
bottomSheet: MaterialButton(onPressed: () {
setState(() {});
}, child: Container(height: 50, color: Colors.green)),
bottomNavigationBar: BottomNavigationBar(
items: const <BottomNavigationBarItem>[
BottomNavigationBarItem(
icon: Icon(Icons.home),
title: Text('Home'),
),
BottomNavigationBarItem(
icon: Icon(Icons.business),
title: Text('Business'),
),
BottomNavigationBarItem(
icon: Icon(Icons.school),
title: Text('School'),
),
],
currentIndex: _selectedIndex,
selectedItemColor: Colors.amber[800],
onTap: _onItemTapped,
),
);
}
}
You should be able to reproduce the behavior i'm talking about when copy/pasting and running the code above.
I'm not sure what i'm doing wrong here, but this appears quite fishy to me.