In JavaFX, if you want to load the a fxml file, then use the FXMLLoader as,
FXMLLoader.load(getClass().getResource("application/sample.fxml"));
To load stylesheet use as.
scene.getStlyeShees().add(getClass().getResource("application/sample.css").toExternalForm);
In your code, you are adding the fxml file in the stylesheets list of the scene which is wrong. Try using FXMLLoader as said above to load your fxml file.
Refer docs: http://docs.oracle.com/javafx/2/get_started/fxml_tutorial.htm