How to set border radius to bottom app bar in a fl

I want to set the borderRadius to an Bottom Navigation App Bar as its shown in the image. I have tried to put Bottom Navigation App Bar to a ClipRRect borderRadius and in a Container decoration but it didn't worked. So how can I apply the topLeft, and topRight border radius to my bottom navigation bar. Kindly help to let me know how can I do it?

main.dart

    void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Food Ordering',
      theme: ThemeData(primarySwatch: Colors.blue, primaryColor: Colors.white),
      home: MyStatefulWidget(),
      routes: <String, WidgetBuilder>{
        '/detail-page': (BuildContext context) => MyDetailPage(),
      },
    );
  }
}

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 List<Widget> _widgetOptions = <Widget>[
    HomePage(),
    HomePage(),
    HomePage(),
    HomePage(),
  ];

  void _onItemTapped(int index) {
    setState(() {
      _selectedIndex = index;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Container(
        child: _widgetOptions.elementAt(_selectedIndex),
      ),
      bottomNavigationBar: BottomNavigationBar(
          items: <BottomNavigationBarItem>[
            BottomNavigationBarItem(
              icon: Image.asset('assets/icon-home.png'),
              title: Text('Home'),
            ),
            BottomNavigationBarItem(
              icon: Image.asset('assets/icon-mentors.png'),
              title: Text('Mentors'),
            ),
            BottomNavigationBarItem(
              icon: Image.asset('assets/icon-messages.png'),
              title: Text('Messages'),
            ),
            BottomNavigationBarItem(
              icon: Image.asset('assets/icon-settings.png'),
              title: Text('Settings'),
            ),
          ],
          currentIndex: _selectedIndex,
          selectedItemColor: Colors.blue,
          onTap: _onItemTapped),
    );
  }
}

Put it inside a stack. Don't add the Bottom Navigation Bar to the scaffold directly.

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Some Text'),
      ),
      body: Stack(
        children: <Widget>[
          bodyContent,
          Positioned(
            left: 0,
            right: 0,
            bottom: 0,
            child: bottomNavigationBar,
          ),
        ],
      ),
    );
  }

  Widget get bodyContent {
    return Container(color: Colors.red);
  }

  Widget get bottomNavigationBar {
    return ClipRRect(
      borderRadius: BorderRadius.only(
        topRight: Radius.circular(40),
        topLeft: Radius.circular(40),
      ),
      child: BottomNavigationBar(
        items: [
          BottomNavigationBarItem(icon: Icon(Icons.home), title: Text('1')),
          BottomNavigationBarItem(icon: Icon(Icons.usb), title: Text('2')),
          BottomNavigationBarItem(
              icon: Icon(Icons.assignment_ind), title: Text('3')),
          BottomNavigationBarItem(
              icon: Icon(Icons.multiline_chart), title: Text('4')),
        ],
        unselectedItemColor: Colors.grey,
        selectedItemColor: Colors.black,
        showUnselectedLabels: true,
      ),
    );
  }
}

Output