No TabController for TabBar.
As The Error is Stating you need - TabController Edit Your Code as Below.
@override
Widget build(BuildContext context) {
// TODO: implement build
return DefaultTabController( // Added
length: 4, // Added
initialIndex: 0, //Added
child: Scaffold(
appBar: AppBar(
title: new Text("Tab App."),
),
drawer: sideDrawer(context), // Passed BuildContext in function.
bottomNavigationBar: bottomTabBar()),
);
}
Widget sideDrawer(BuildContext context){ ......