flutter snackbar error

  @override
  Widget build(BuildContext context) {
    final Size screenSize = MediaQuery.of(context).size;

    return new Scaffold(
      appBar: new AppBar(
        title: new Text('Greenbook'),
      ),
      body: ...,
      floatingActionButton: new FloatingActionButton(
        onPressed: () {
          if (this._formKey.currentState.validate()) {
            Scaffold.of(context).showSnackBar(new SnackBar(
              content: new Text("Sending Message"),
            ));
            this.submit();
          }
        },
        tooltip: 'Calcular',
        child: new Icon(Icons.done),
      ),
    );
  }