Wrong dialog activation

#include <QtWidgets>
class Dialog: public QDialog
{
public:
    Dialog(QWidget *parent) : QDialog(parent, Qt::Window)
    {
        setWindowTitle(tr("sub dialog"));
    }

    void mousePressEvent(QMouseEvent *)
    {
        QColorDialog::getColor();
    }

};

class MainDialog : public QMainWindow
{
public:
    MainDialog()
    {
        setWindowTitle(tr("main dialog"));
    }

    void mousePressEvent(QMouseEvent *)
    {
        Dialog *dlg = new Dialog(this);
        dlg->show();
    }

};

int main(int argc, char **argv)
{
QApplication app(argc, argv);
MainDialog dlg;
dlg.show();
return app.exec();
}