Flutter showSnackBar called on null

I am trying to show a snack bar in a tabbed flutter app as follows:

  var scaffoldKey = new GlobalKey<ScaffoldState>();
    scaffoldKey.currentState
      .showSnackBar(new SnackBar(
        content: new Text("This is a message")
      );
  });

But I am getting the error:

NoSuchMethodError: The method 'showSnackBar' was called on null.

In the main.dart file I initialize the app as follows:

runApp(new MaterialApp(...));

And then the snackBar is being called in home.dart which has a tabbed interface:

Widget build (BuildContext context) => new Scaffold(…);

So I'm a bit confused on which scaffold to call the snackbar on, and how, and also why scaffoldKey.currentState is null.