Render windget to QPixmap

[giker]

I have application whith whidgets on mainform. I need to render one of widgets (qwtPlot) to QPixmap using QGraphicsScene. I use such code for that:

QPixmap pixmap(300, 300);
QPainter painter(&pixmap);
painter.fillRect(0, 0, 300, 300, Qt::black);
QGraphicsScene *scene = new QGraphicsScene();
QGraphicsProxyWidget *widget = scene->addWidget(qwtPlot);
qwtPlot->repaint();
qwtPlot->show();
scene->render(&painter, QRectF(), QRect(), Qt::IgnoreAspectRatio);
if (pixmap.save(QApplication::applicationDirPath() + "/image.png", "PNG"))
{  
    QMessageBox box;
    box.setText("Saved");
    box.exec();
}
delete widget;
delete scene;

QPixmap pixmap(300, 300);
QPainter painter(&pixmap);
painter.fillRect(0, 0, 300, 300, Qt::black);
QGraphicsScene *scene = new QGraphicsScene();
QGraphicsProxyWidget *widget = scene->addWidget(qwtPlot);
qwtPlot->repaint();
qwtPlot->show();
scene->render(&painter, QRectF(), QRect(), Qt::IgnoreAspectRatio);
if (pixmap.save(QApplication::applicationDirPath() + "/image.png", "PNG"))
{  
    QMessageBox box;
    box.setText("Saved");
    box.exec();
}
delete widget;
delete scene;

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But I haven't any whidgets in "image.png" file - all file is black. Why my widget don't show in the file?