How to add a robot and light object in GUI

i am trying to add a button in GUI which when you press it with the mouse adds a robot and another button which when you press it adds a light object. I have worked the code out and i do not get an error but the buttons are not working.

RobotArena(double xS, double yS){
        xSize = xS;
        ySize = yS;
        allItems = new ArrayList<ArenaItem>();                  
        allItems.add(new Robot (xS/2, yS/3, 15, 20, 5));
        allItems.add(new Obstacle (20, 20, 20, 30, 10));

        }

public void addRobot(double x, double y) {
        allItems.add(new Robot(x, y, 10, 45, 10));  
        System.out.println(allItems);

    }

GUI:

Button btnAdd = new Button("Robot");                    
            btnAdd.setOnAction(new EventHandler<ActionEvent>() {    
                @Override
                public void handle(ActionEvent event) {
                    arena.addRobot(20,40);

               }
            });

Light class:
public Light (double ix, double iy, double ir, double an, double sp) {
        super(ix, iy, ir, an, sp);
        col = 'y';
    }

RobotArena:
public void AddLight(double ix, double iy, double ir, double an, double sp) {
        allItems.add(new Light(ix, iy, ir, an, sp));

    }

GUI:
Button btnLight = new Button("Light");                  
            btnAdd.setOnAction(new EventHandler<ActionEvent>() {    
                @Override
                public void handle(ActionEvent event) {

               }
            });

I would really appreciate your help!