Qt 4.7+Xlib crash on QWidget::winId() method

Question

Sorry for my english, but I have the next problem. I am writing a window manager using Qt 4.7 and Xlib. I have class Manager that inherits QApplication and reimplemented method X11EventFilter in it. In X11EventFilter method I catch necessary events from XServer. When I receive MapRequest event, I catch appearing of new window and reparent it to my own widget. And when I create that widget and call QWidget::show() or QWidget::winId() methods, program crashes. What is the problem?

Here is a method where widget is creating. I wonder, when this function calls few times on start of program, everything is OK.

void Manager::createClientWindow(Qt::HANDLE pWinID)
{
    QMWindowWidget *lWindowWidget = new QMWindowWidget(pWinID);
    /*some code*/
    lWindowWidget->show();//crash is here
    Qt::HANDLE widgetId = lWindowWidget->winId();//and here
    /*some code*/
}

Here is a x11EventFilter method where createClientWindow function is called

bool Manager::x11EventFilter(XEvent *pEvent)
{
    switch(pEvent.type)
    {
    /*some code*/
    case MapRequest:
    {
        Qt::HANDLE lWindow = pEvent->xmaprequest.window;

        QMWindowWidget* lWidget = findWidget(lWindow);
        if (!lWidget)
        {
            lWidget = dynamic_cast<QMWindowWidget*>(QWidget::find(lWindow));
        }

        if (lWidget)
        {
            XMapWindow(QX11Info::display(), lWindow);
            lWidget->show();
            XRaiseWindow(QX11Info::display(), lWidget->winId());
            return true;
        }
        else
        {
            createClientWindow(lWindow);//here is where function is called
            return true;
        }
    }
    break;
    /*some code*/
    }    //switch
    return false;
}

Answer 1

Problem is resolved! I paste this two strings before QApplication::exec()

XClearWindow(QX11Info::display(), QX11Info::appRootWindow());
XSync(QX11Info::display(), false);