FlatButton onPressed Not Working In Positioned Widget

Question

Whenever I used FlatButton onPressed Method Is Not Working In Positioned Widget.

I am designing one screen in that buttons are always in the bottom of the screen and that button click we can move to other screens,but here it is not working, can anyone check and help me out.thaks in advance

           child: Stack(
               // height: MediaQuery.of(context).size.height,
               children: <Widget>[
                     Positioned(
                         left: 0,
                         bottom: 0,
                           // alignment: Alignment(0, 0),
                           child: Row(
                             mainAxisSize: MainAxisSize.max,
                             crossAxisAlignment: CrossAxisAlignment.end,
                             mainAxisAlignment: MainAxisAlignment.end,
                             children: <Widget>[
                               SizedBox(height: 115),
                               ButtonTheme(
                                 minWidth:
                                     MediaQuery.of(context).size.width / 2,
                                 height: 60.0,
                                 child: FlatButton.icon(
                                   icon: Icon(FontAwesomeIcons.fileInvoice),
                                   color: Colors.white,
                                   onPressed: () {
                                  MaterialPageRoute(builder: (context) => MainScreen());
                                   },
                                   label: Text(
                                     "My Orders",
                                     textAlign: TextAlign.center,
                                     style: TextStyle(
                                       // decoration: TextDecoration.underline,
                                       fontSize: 25,
                                       fontWeight: FontWeight.bold,
                                       color: Theme.of(context).accentColor,
                                     ),
                                   ),
                                 ),
                               ),
                               SizedBox(height: 115),
                               ButtonTheme(
                                 minWidth:
                                     MediaQuery.of(context).size.width / 2,
                                 height: 60.0,
                                 child: FlatButton.icon(
                                   icon: Icon(FontAwesomeIcons.dollarSign),
                                   color: Colors.white60,
                                   onPressed: () {
                                    Navigator.pushReplacementNamed(
                                       context, '/home');
                                   },
                                   label: Text(
                                     "Bean Balnace",
                                     textAlign: TextAlign.center,
                                     style: TextStyle(
                                       // decoration: TextDecoration.underline,
                                       fontWeight: FontWeight.bold,
                                       fontSize: 20,
                                       color: Theme.of(context).accentColor,
                                     ),
                                   ),
                                 ),
                               ),
                             ],
                           )),

Answer 1

Change this line:

MaterialPageRoute(builder: (context) => MainScreen());

to this:

Navigator.push(context, MaterialPageRoute(builder: (context) => MainScreen()));