QGraphicsDropShadowEffect and winId() weird behaviour

MainWindow::MainWindow()
{
	QGraphicsDropShadowEffect * effect = new QGraphicsDropShadowEffect;
	effect->setBlurRadius(2);
	effect->setOffset(0, 5);
	effect->setColor(QColor::fromRgbF(0, 0, 0, 0.25));

	QToolBar *toolbar = new QToolBar;
	toolbar->setMovable(false);
	toolbar->setFloatable(false);
	toolbar->setAutoFillBackground(true);
	toolbar->setGraphicsEffect(effect);	// want a shadow under toolbar (1)
	
	QToolButton *tb1 = new QToolButton;
	tb1->setText("Button 1");
	
	QToolButton *tb2 = new QToolButton;
	tb2->setText("Button 2");
	
	QToolButton *tb3 = new QToolButton;
	tb3->setText("Button 3");

	QWidget *widg = new QWidget;
	widg->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
	QHBoxLayout *lay = new QHBoxLayout;
	lay->setContentsMargins(0,0,0,0);
	lay->addWidget(tb1);
	lay->addWidget(tb2);
	lay->addStretch(1);
	lay->addWidget(tb3);
	widg->setLayout(lay);
	toolbar->addWidget(widg);

	addToolBar(Qt::TopToolBarArea, toolbar);


	QFrame *wnd = new QFrame;	
	wnd->winId();	// i need a win id (2)
	setCentralWidget(wnd);

	//resize(800, 600);
}