Console gets spammed with a MissingReferenceException after a terrain with a modified custom material is deleted from the scene.

Repro steps:
1. Open Scene1 from attached project
2. Select Terrain in the scene view
3. In the inspector, change the assigned material (see attached gif)
4. Delete terrain from scene.
5. Observe errors in console.

Expected outcome: no error spam.

Notes:
- regression, could not repro in 5.5.0p3