QVTKInteractor cannot control the event loop

With a vtkGenericOpenGLRenderWindow, I am trying to use the interactor style vtkInteractorStyleTrackballCamera along with vtkCallbackCommand and added an observer to vtkDistanceWidget.

I see that the vtkCallbackCommand is not executed and I get the error message “QVTKInteractor cannot control the event loop”.

What might be wrong ?