Crash when calling winID() on window handle

#include "mainwindow.h"
#include "./ui_mainwindow.h"

#include <QPushButton>
#include <QWindow>
#include <QMessageBox>

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    QObject::connect(ui->btnGetParent, SIGNAL(clicked()), this, SLOT(btnGetParent_click()));
}

void MainWindow::btnGetParent_click()
{
    WId parentId;

    parentId = this->windowHandle()->parent()->winId();

    char buffer[100];
    sprintf(buffer, "Parent window: %d", (long) parentId);

    QMessageBox::information(this, "Info", QString(buffer));
}

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

parentId = this->windowHandle()->parent()->winId();