Why when passing an object between classes it becomes null?

I am developing a Java application.

I have created a GUI for creating an object, but I want to begin constructing it, then pass it to other GUI to finish the construction, before storing it in the database.

This is a class that I call the Selector:

    private void aceptar() throws Exception {
        Comprobante comprobanteOperaciones = new Comprobante();
        if (partidas.size() > 0) {
            comprobanteOperaciones.setPartidas(partidas);
            comprobanteOperaciones.setFechaOperaciones(dialog.getTxtFechaOperaciones().getDate());
            comprobanteOperaciones.setDescripcion(generarDescripcion());
            new ComprobanteOperacionesRegistrarController(dialog, securityFacade, comprobanteOperaciones).openDialog();
        } else {
            throw new Exception("No hay partidas seleccionadas");
        }
    }

Then I create the object and I set some of the attributes, before sending it to the other class, that I call Register:

    public class ComprobanteOperacionesRegistrarController{

    private Comprobante comprobanteOperaciones;

    public ComprobanteOperacionesRegistrarController(Window window,
                                                     SeguridadFacade seguridadFacade, Comprobante comprobante) throws Exception {
        super(new UI_ComprobanteOperacionesRegistrar(window), seguridadFacade);
        this.comprobanteOperaciones = comprobante;
    }

    @Override
    protected void initialize() throws Exception {
        operacionesFacade = OperacionesFacade.getInstance();
        configuracionFacade = ConfiguracionFacade.getInstance();
        reportesFacade = ReportesFacade.getInstance();
        tableOperacionesModel = (DefaultTableModel) dialog.getTableOperaciones().getModel();

        dialog.getBtnCancelar().addActionListener(this);
        dialog.getBtnImprimir().addActionListener(this);
        dialog.getTxtFechaOperaciones().addPropertyChangeListener(
                new PropertyChangeListener() {
                    @Override
                    public void propertyChange(PropertyChangeEvent propertyChangeEvent) {
                        llenarPartidasParcial();
                    }
                }
        );

        llenarNoComprobante();
        llenarDatosComprobante();
        llenarPartidasParcial();
    }
...

When I debug the app, I added breakpoints before and after sending the object to the other class.

Before sending the object, it is filled with the information and the object is created. When I go to the next breakpoint:

this.comprobanteOperaciones = comprobante;

But when I evaluate the object there, it's null.

Breakpointed line is not executed yet, so the assignment is not happened. Add some code to below of the assigment and add breakpoint there

So I fixed it by calling the methods that fill information in the constructor, after the object was initialized:

public class ComprobanteOperacionesRegistrarController{

    private Comprobante comprobanteOperaciones;

    public ComprobanteOperacionesRegistrarController(Window window,
                                                     SeguridadFacade seguridadFacade, Comprobante comprobante) throws Exception {
        super(new UI_ComprobanteOperacionesRegistrar(window), seguridadFacade);
        this.comprobanteOperaciones = comprobante;

        llenarNoComprobante();
        llenarDatosComprobante();
        llenarPartidasParcial();
    }

    @Override
    protected void initialize() throws Exception {
        operacionesFacade = OperacionesFacade.getInstance();
        configuracionFacade = ConfiguracionFacade.getInstance();
        reportesFacade = ReportesFacade.getInstance();
        tableOperacionesModel = (DefaultTableModel) dialog.getTableOperaciones().getModel();

        dialog.getBtnCancelar().addActionListener(this);
        dialog.getBtnImprimir().addActionListener(this);
        dialog.getTxtFechaOperaciones().addPropertyChangeListener(
                new PropertyChangeListener() {
                    @Override
                    public void propertyChange(PropertyChangeEvent propertyChangeEvent) {
                        llenarPartidasParcial();
                    }
                }
        );
    }
...

Given that super() method in the Abstract parent class, calls the initialize(), so I was trying to fill up data into an object that was not initialized and was null yet. This fixed it for me.