created errocode.fxml
need to do
@FXML
private Label errocodelabel;

then in code do this
       Stage popup = new Stage();
        Parent root = FXMLLoader.load(getClass().getResource("view/Errorcode.fxml"));
        Scene popup_scene = new Scene(root);
        popup.setTitle("Errorcode");
	errorcodelabel.setText("(4096) Donut overflow");
        popup.setScene(popup_scene);
        popup.show();