QGLWidget doesn't emit signal to QMainWindow

Question

EDIT: Okay I solved the problem myself, this is humiliating... There was no instance of MainWindow created in py main() but I used the Widget itself as window -.-

I'm playing around a bit with OpenGL in Qt using the QGLWidget (Qt Version 5.3). My problem is the following: I have a class PaintWidget that derives from QGLWidget. In its PaintWidget::mousePressEvent(QMouseEvent*) I read some value from the stencil buffer and want to send this via an emit to the MainWindow. The signal is implemented as

class PaintWidget : public QGLWidget {
    Q_OBJECT
signals:
    void objectClicked(int id);

public:
    PaintWidget(QWidget* parent = 0);
...
}

and the mousePressEvent method look

void PaintWidget::mousePressEvent(QMouseEvent* event) {   
    GLuint id;
    glReadPixels(event->x(), m_height - event->y(), 1, 1, GL_STENCIL_INDEX, GL_UNSIGNED_INT, &id);
    if (id > 0) {
        emit objectClicked((int)id);
    }
}

Now when I connect this signal to a slot inside of PaintWidget everything works as expected, meaning the slot is called. But when I connect it to some slot of my MainWindow it doesn't work. The MainWindow's constructor is:

MainWindow::MainWindow(QWidget* parent) : QMainWindow(parent) {
    m_paintEngine = PaintEngine::instance();
    m_paintWidget = new PaintWidget();
    connect(m_paintWidget, SIGNAL(objectClicked(int)), this, SLOT(dummy(int)));
    setCentralWidget(m_paintWidget);
}

Here, dummy is a public slot and it is newer called. Why doesn't this work?

Thanks, Michael