There should be a way to visualize models without any material assigned

From tooling perspective, it would be nice to be able to somehow visualize also models that have no materials assigned. Otherwise it's easy to accidentally leave invisible models lying around in a complex scene when you delete a material that is used by many models.

E.g. Use some kind of obvious default "error" material or just use default material if no material is specified. If this is not good default behavior, make it possible to enable this somehow. E.g. via some private API call or maybe SceneEnvironment could have "defaultMaterial" property?