how to get listview button to go to random route onTap

I am trying to use solution from here How make button which open random page in Flutter? but instead of RaisedButton, do it with my Listview buttons.

Here's what I have so far after I added the RandomRouteGenerator from the other StackOverflow:

ListView(
      children: const <Widget>[
        Card(
          child: ListTile(
            leading: Text('🐔'),
            title: Text('Chicken'),
            onTap: () {
              Navigator.of(context).pushNamed(
                RouteGenerator.getRandomNameOfRoute());
            },
          ),
        ),

//... more Cards that would become other listview items

Here's the errors:

Compiler message:
lib/main.dart:163:28: Error: Not a constant expression.
              Navigator.of(context).pushNamed(
                           ^^^^^^^
lib/main.dart:163:25: Error: Method invocation is not a constant expression.
              Navigator.of(context).pushNamed(
                        ^^
lib/main.dart:164:32: Error: Method invocation is not a constant expression.
                RouteGenerator.getRandomNameOfRoute());
                               ^^^^^^^^^^^^^^^^^^^^
lib/main.dart:163:37: Error: Method invocation is not a constant expression.
              Navigator.of(context).pushNamed(
                                    ^^^^^^^^^
lib/main.dart:162:20: Error: Not a constant expression.
            onTap: () {
                   ^^

Thanks