Adding Observer within c++ loadable crashes Slicer

Hello all!
So I was trying to implement this line of code from Python in c++:

id = crosshairNode.AddObserver(slicer.vtkMRMLCrosshairNode.CursorPositionModifiedEvent, onMouseMoved)

and my first guess was this one:

  vtkNew<vtkMRMLScene> scene;
    vtkMRMLVolumeNode* volumeNode = vtkMRMLVolumeNode :: SafeDownCast( scene->GetNodeByID("MRHead"));
    vtkMRMLVolumeNode* inputVolumeNode;
    vtkMRMLCrosshairNode* crosshairNode;
    vtkSmartPointer<vtkCallbackCommand> callback;
   crosshairNode -> AddObserver( vtkMRMLCrosshairNode:: CursorPositionModifiedEvent, callback)

this crashes my application immediately.

I also tried to add an observer this way, which actually did not crash, but I don’t know if the observer was actually listening.

I tried to create an event handler that had the NodeCallback Type:

vtkNew<vtkMRMLCoreTestingUtilities::vtkMRMLNodeCallback> eventHandler() {
 vtkNew<vtkMRMLCoreTestingUtilities::vtkMRMLNodeCallback> callback;
 qDebug() << "... handler";
 return callback;
      scene->AddObserver(vtkCommand :: AnyEvent, eventHandler());

I woul be really glad for any help.