custom QPGraphicsScene class not shown in QGraphicsView

I tried to create a custom board background for 2048, when I ran it in Debugger Mode, gbp points to this line "for (int row = 0; row < SIZE; ++row)", which is very confusing for me

#include "guiboard.hh"

guiboard::guiboard(QObject* parent)
        : QGraphicsScene(parent)
{
    setSceneRect(50, 200, BOARD_SIZE, BOARD_SIZE);
}

void guiboard::make_bg()
{
    QPixmap image(QString::fromStdString("wood.jpg"));

    // Scaling the pixmap
    image = image.scaled(BOARD_SIZE, BOARD_SIZE);
    addPixmap(image);

    QPen pen;
    pen.setBrush(Qt::gray); pen.setWidth(3);

    for (int row = 0; row < SIZE; ++row)
    {
        for (int col = 0; col < SIZE; ++col)
        {
            int ax = 50 + SPACING + row * SPACING + row * TILE_SIZE;
            int ay = 200 + SPACING + col * SPACING + col * TILE_SIZE;
            addRect( ax, ay, TILE_SIZE, TILE_SIZE, pen, QColor::fromRgb(0, 0, 0));
        }
    }
}

Here is the MainWindow

#include "mainwindow.hh"
#include "ui_mainwindow.h"
#include <QGraphicsView>

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
    , gameboard()
{
    ui->setupUi(this);
    ui->stackedWidget->setCurrentIndex(1);
    ui->boardDM->setGeometry(50, 200, 500, 500);

    gameboard->make_bg();
    ui->boardDM->setScene(lauta);
}

MainWindow::~MainWindow()
{
    delete ui;
}

(I'm on my baby steps to learn C++, so detailed explanation is much appreciated. Thank you!)