#include <QApplication>
#include <QDockWidget>
#include <QMainWindow>
#include <QtWebKitWidgets/QWebView>
#include <QTimer>

class DoCrash : public QObject{
    Q_OBJECT
public:
    DoCrash( QDockWidget* widget ) :m_dockWidget( widget ){}
public slots:
    void startCrach(){
        m_dockWidget->setFloating( true );
        QTimer::singleShot( 100, this, SLOT(doCrash()) );
    }
private slots:
    void doCrash(){
        m_dockWidget->setFloating( false );
    }
private:
    QDockWidget* m_dockWidget;
};


int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QMainWindow mainWindow;
    QDockWidget* dockWidget = new QDockWidget( &mainWindow );
    dockWidget->setFeatures( QDockWidget::DockWidgetFloatable | QDockWidget::DockWidgetMovable );
    QWebView* view = new QWebView( dockWidget );
    view->load( QUrl("http://www.google.com") );
    dockWidget->setWidget( view );
    mainWindow.addDockWidget( Qt::BottomDockWidgetArea, dockWidget );
    DoCrash crash( dockWidget );
    mainWindow.show();

    QTimer::singleShot( 10, &crash, SLOT(startCrach()) );

    return a.exec();
}

class QWebPagePrivate{
  ....
  QWindow* window;  -> QPointer< QWindow > window; // It's work.
};