calling beams as part of "parts" generates error

Hi Jacques,

I have built a visualisation of a car with turning wheels. This used to work with a previous version of SimPyLC, but with the latest version it generates the error
 File "H:\Mijn Documenten\TINRES\simulations\test\visualisation.py", line 9, in <lambda>
   return Cylinder.__call__(self, shift = shift, rotation = rotation, parts = lambda: self.spoke() + self.spoke(rotation = 90))
 File "C:\Python36\lib\site-packages\SimPyLC\scene.py", line 217, in __call__
   if self.scene._displayMode == Scene._dmCheck:
AttributeError: 'Beam' object has no attribute 'scene'


What I do is define my own class Wheel, of which I use four instances:
class Wheel (Cylinder):
   def __init__(self, center = (0, 0, 0)):
       Cylinder.__init__ (self, size = (1, 1, 0.2), center = center, color = (0, 1, 0), axis = (0, 1, 0), angle = 90, pivot = (1, 0, 0))
       self.spoke = Beam(size = (0.3, 0.1, 0.9), color = (0, 1, 0), pivot = (1, 0, 0))

   def __call__(self, shift = (0, 0, 0), rotation = 0):
       return Cylinder.__call__(self, shift = shift, rotation = rotation, parts = lambda: self.spoke() + self.spoke(rotation = 90))


The spokes of the wheel are beams, but this now no longer works. Is this a bug or did I make a mistake? Please find a complete example of the problem here: med.hr.nl/berwm/demo.zip

Thanks for your reply,

Wouter