The class ContourModelGLMapper2DBase causes vtkWarnings to be triggered when rendering the contours.
The reason for the warning is that during at the begining of the rendering-pipeline the vtkRendererCollection determines the number of layers used.
After that it calls Render() on all the renderers.
In the Render() of the 'ContourModelGLMapper2DBase' the method DrawContour() is called that adds additional renderers.
This is bad as the vtkRendererCollection cannot adapt the number of layers accordingly.
Calling Render() on a renderer should not add additional renderers!