How to get IndexedStack children state flutter

I used IndexedStack with BottomNavigationBar to keep state of Screens/pages/Widget.

But now if I navigate to second or third page from home tab then I'm not able to get the current state of page. Basically I want to dispose the video controller but when I changed the tab, onDispose() method is not called in my VideoPlayer class.

Code to implement bottom tab bar

int _selectedIndex = 0;
final _classesScreen = GlobalKey<NavigatorState>();
final _programScreen = GlobalKey<NavigatorState>();
final _scheduleScreen = GlobalKey<NavigatorState>();
final _profileScreen = GlobalKey<NavigatorState>();

@override
Widget build(BuildContext context) {
return Scaffold(
  body: IndexedStack(
    index: _selectedIndex,
    children: <Widget>[
      Navigator(
        key: _classesScreen,
        onGenerateRoute: (route) => MaterialPageRoute(
          settings: route,
          builder: (context) => FitClasses(,
          ),
        ),
      ),
      Navigator(
        key: _programScreen,
        onGenerateRoute: (route) => MaterialPageRoute(
          settings: route,
          builder: (context) => Programs(),
        ),
      ),
      Navigator(
        key: _scheduleScreen,
        onGenerateRoute: (route) => MaterialPageRoute(
          settings: route,
          builder: (context) => Schedule(),
        ),
      ),
      Navigator(
        key: _profileScreen,
        onGenerateRoute: (route) => MaterialPageRoute(
          settings: route,
          builder: (context) => Profile(),
        ),
      ),
    ],
  ),
  bottomNavigationBar: Theme(
    data: Theme.of(context).copyWith(
        canvasColor: Colors.white,
        splashColor: Color(0xffFFED31),
        unselectedWidgetColor: Colors.white,
        primaryColor: Color(0xffFFED31),
        backgroundColor: Color(0xff3A3A3A),
        textTheme: Theme.of(context)
            .textTheme
            .copyWith(caption: new TextStyle(color: Colors.white))),
    child: BottomNavigationBar(
      items: <BottomNavigationBarItem>[
        BottomNavigationBarItem(
          icon: Container(
              height: 20.0,
              width: 20.0,
              child: _selectedIndex == 0
                  ? SvgPicture.asset(AppConfig.classes,
                      color: Color(0xffFFED31), semanticsLabel: 'classes')
                  : SvgPicture.asset(AppConfig.classes,
                      color: Colors.white, semanticsLabel: 'classes')),
          title: Text(
            'Classes',
            style: TextStyle(fontSize: 10),
          ),
        ),
        BottomNavigationBarItem(
          icon: Container(
              height: 20.0,
              width: 20.0,
              child: _selectedIndex == 1
                  ? SvgPicture.asset(AppConfig.programs,
                      color: Color(0xffFFED31), semanticsLabel: 'programs')
                  : SvgPicture.asset(AppConfig.programs,
                      color: Colors.white, semanticsLabel: 'programs')),
          title: Text(
            'Programs',
            style: TextStyle(fontSize: 10),
          ),
        ),
        BottomNavigationBarItem(
          icon: Container(
              height: 20.0,
              width: 20.0,
              child: _selectedIndex == 2
                  ? SvgPicture.asset(AppConfig.schedule,
                      color: Color(0xffFFED31), semanticsLabel: 'schedule')
                  : SvgPicture.asset(AppConfig.schedule,
                      color: Colors.white, semanticsLabel: 'schedule')),
          title: Text(
            'Schedule',
            style: TextStyle(fontSize: 10),
          ),
        ),
        BottomNavigationBarItem(
          icon: Container(
              height: 20.0,
              width: 20.0,
              child: _selectedIndex == 3
                  ? SvgPicture.asset(AppConfig.profileIcon,
                      color: Color(0xffFFED31), semanticsLabel: 'profile')
                  : SvgPicture.asset(AppConfig.profileIcon,
                      color: Colors.white, semanticsLabel: 'profile')),
          title: Text(
            'Profile',
            style: TextStyle(fontSize: 10),
          ),
        ),
      ],
      currentIndex: _selectedIndex,
      onTap: (val) => _onTap(val, context),
      type: BottomNavigationBarType.fixed,
      backgroundColor: Color(0xff3A3A3A),
      /*selectedItemColor: Color(0xffFFED31),
      selectedIconTheme: IconThemeData(color: Color(0xffFFED31)),
      unselectedIconTheme: IconThemeData(color: Color(0xffFFFFFF)),*/
      selectedLabelStyle: TextStyle(fontSize: 10, color: Color(0xffFFED31)),
      unselectedLabelStyle:
          TextStyle(fontSize: 10, color: Color(0xffFFFFFF)),
      ),
    ),
  );
}

 void _onTap(int val, BuildContext context) {
  if (_selectedIndex == val) {
   switch (val) {
     case 0:
       _classesScreen.currentState.popUntil((route) => route.isFirst);
       break;
     case 1:
       _programScreen.currentState.popUntil((route) => route.isFirst);
       break;
     case 2:
       _scheduleScreen.currentState.popUntil((route) => route.isFirst);
       break;
     case 3:
       _profileScreen.currentState.popUntil((route) => route.isFirst);
       break;

     default:
   }
 } else {
   if (mounted) {
     setState(() {
       _selectedIndex = val;
     });
   }
 }
}

Please watch this video to understanding the my point