Common cases of this are when python references an object from the AddObject actuator that has ended, or a scene has been loaded and the old objects freed.
This means some scripts will raise errors now in certain cases but better give the error early rather then failing silently with strange hard to track down behavior & crashes.
Added "isValid" attribute for checking objects are in a scene.
At the moment it uses the SceneGraph Node to check of the objects valid but it might be better to do this in a more generic way so scenes, meshes etc also have this check.
e8f4d93221