All child widgets not rendered to PDF

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent)
{

    widget = new QWidget;
    QGridLayout *layout = new QGridLayout(widget);
    layout->addWidget( new QRadioButton("Radio button"), 0,0 );
    layout->addWidget( new QLabel("Label"), 0,1 );
    layout->addWidget( new QPushButton( "Button 1" ), 1,0 );
    layout->addWidget( new QPushButton("Button 2"), 1,1 );

    setCentralWidget(widget);
    QTimer::singleShot(2000, this, SLOT(pic()));
}

void MainWindow::pic()
{
    qDebug("MainWindow::pic()");

    QPicture pic;
    widget->render(&pic); // render to QPicture
    QPrinter printer;
    printer.setOutputFileName( "test.pdf" );
    QPainter painter(&printer);
    pic.play(&painter); // render QPicture to printer.
}