Could not find a generator for route

Because Of instantiated two MaterialApp widgets. You need to remove the one in MyApp class and may change it to Scaffold

Example:

class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return new Scaffold(
// ...                    
                return new ListTile(
                  onTap: () {                                         
                    Navigator.pushNamed(context, "/listadecompras");
                  },
// ...
}

THE PROBLEM IS YOUR CODE IS – the route is trying to resolve for the nearest MaterialApp which has no route definition. That said you should use only one MaterialApp as the root of your widget tree.