Warning triggered by entity reference field using entity selection mode "Views: Filter by an entity reference view"

Summary: The warning is triggered on the node/edit form when a user makes a selection to an entity reference field that is using the entity selection mode of "Views: Filter by an entity reference view."

To reproduce:
1. Create two entities A and B.
2. Create a view with the "Entity Reference" display that lists entities of type B.
3. Add an entity reference field on A. Under "Entity Selection Mode" choose "Views: Filter by an entity reference view" and select the view from step 2.
4. Create fake content of type B.
5. Create an entity of type A. In the entity reference field, choose one of the B entities. The warning message will appear.
6. Switch the "Entity Selection Mode" to "Simple (with optional filter by bundle)" and re-test step 5. No warning message appears.