Fixed bug that causes crashes when removing objects from the scene.

When objects are added to a scene, *object.material._physijs* is checked to see if it is undefined (line 852). The same is not done when objects are removed which causes "Uncaught TypeError: Cannot read property 'id' of undefined" when *object.material._physijs* is indeed undefined. All I did was add the same check. Incidentally, this bug seems to be 2 months old.