AJAX WSOD when adding empty Relationship

I've the View with no relationships.
How to reproduce the problem:
1. Edit existing View (or create a new one).
2. Add Relationship.
3. You should see the message: 'There are no relationships available to add.'
4. Click any of the buttons.
5. The URL should be: /admin/structure/views/ajax/add-item/view_name/page/relationship
You should see the AJAX WSOD.
See the screenshots.

P.S. Doesn't affect any functionality apart of awful WSOD.