[vtkusers] Bug in vtkInteractorStyle...Actor

[Patrick Neumann]

Hi,
I found a bug in vtkInteractorStyleTrackballActor and
vtkInteractorStyleJoystickActor.

Sometimes InteractionProp is not initalized but used for example to call
GetCenter. The previous call of FindPickedActor not allways set
InteractionProp to a usable value.

You can see this bug for example in the Cone3 demo. If you stay for
example with the mouse not over the object and change the style from
camera to actor the first time you get a segmentation fault.

Regards
--
Patrick Neumann