Calling a QFileDialog static function or QFileDialog::exec() with native dialogs temporarily sets qApp::activeWindow to NULL

#include <QApplication>
#include <QFileDialog>
#include <QVBoxLayout>
#include <QPushButton>
class Dialog : public QDialog
{
    Q_OBJECT
public:
    Dialog()
    {
        QVBoxLayout* layout = new QVBoxLayout();
        QPushButton* button = new QPushButton( "Open popup", this );
        layout->addWidget( button );
        connect( button, SIGNAL(clicked()), this, SLOT(openPopup()) );
    }

public slots:
    void openPopup()
    {
        QWidget* widget = qApp->activeWindow();

        QFileDialog::getExistingDirectory(
                    widget, // pass widget or NULL: same behaviour
                    "toto",
                    "tata" );
    
        // qApp->processEvents();

        QWidget* widget2 = qApp->activeWindow();
        // here widget == this and widget2 == NULL. {color:red}THAT'S THE BUG!{color}
        // If called qApp->processEvents(), widget == widget2 == this
    }
};

int main( int argc, char* argv[] )
{
    QApplication app( argc, argv );
    
    Dialog dlg;
    dlg.show();

    return app.exec();
}