Steps to reproduce:

1. Open the attached project
2. Select 'test' model in the project window and open 'Rig' tab in the model importer
3. Enable 'Optimize Game Objects'
4. Add any bone to 'Extra Transforms to Expose' list
5. Setup animation clip in the 'Animations' tab (or you can use the one that is already created)
6. Create 'Animator Controller' asset and add the animation clip to it
7. Create any game object
8. Drag the 'test' model onto that GO to create it as a child of the newly created object
9. Attach created animator controller to the 'test' object 'Animator' component
10. Play the scene
11. Notice the errors are spammed in the console

Note: If you make the 'test' object invisible (that is, hide the game view, open the scene view and drag the camera so that you can not see the object) the errors stop until you focus on the game object in the scene or open the game view again. This also does not happen if the 'test' game object is not set to be a child of any other object.