Instance of FXML controller in JavaFX returns null

Question

I want to create a reference for the FXML Controller by this line in Class Main

Controller controller = (Controller) loader.getController();

to have access to a Controller methode from the Class GrblListener.

Main.controller.updateCurrentPosition(input);

But I always get the error

Exception in thread "EventThread COM5" java.lang.NullPointerException

What's wrong?

Class Main:

public class Main extends Application {

    public static Stage stage;
    public static Controller controller;

    @Override
    public void start(Stage stage) throws Exception {

        FXMLLoader loader = new FXMLLoader(getClass().getResource("Scene.fxml"));
        Parent root = (Parent) loader.load();
        Controller controller = (Controller) loader.getController();

        Scene scene = new Scene(root);
        scene.getStylesheets().add(getClass().getResource("css/default.css").toExternalForm());

        stage.setTitle("...");
        stage.setScene(scene);
        stage.show();

        this.stage = stage;
    }

    public static void main(String[] args) {
        launch(args);
    }
}

Class GrblListener:

class GrblListener implements SerialPortEventListener {

    @Override
    public void serialEvent(SerialPortEvent event) {

        if(event.isRXCHAR() && event.getEventValue() > 0){

            try {
                String input = GrblSender.serialPort.readString();
                System.out.println(input.trim());

                Main.controller.updateCurrentPosition(input);
            }
                catch (SerialPortException ex) {
                System.out.println(ex);
            }
        }
    }
}

Answer 1

You are declaring a local variable in start() , and initializing it:

Controller controller = (Controller) loader.getController();

instead of initializing the static variable you declared:

public static Controller controller ;

public void start(Stage stage) {

    controller = (Controller) loader.getController();

    // ...
}