Steps to repro:

1. Open attached project and 't1' scene;
2. Create Terrain gameobject (GameObject>3D Object>Terrain);
3. In the Inspector select Raise/Lower Terrain tab;
4. Select any brush in the 'Brushes' section;
5. Paint height on terrain by left-clicking on it;
6. In the Lighting window change Lightmapper to Progressive.

Actual result:
Multiple 'Failure computing material mapping for Progressive Lightmapper. Retrying' warnings are shown. See attached video.
Regression introduced in Unity 2018.2.0a1.

Expected result:
No warning messages are shown after changing GI backend.

Notes:
- Reproducible in 2018.2.0a8, 2018.2.0a2, 2018.2.0a1;
- Not reproducible in 2018.1.0f1, 2017.4.1f1;
- Reproducible on Windows 10.
- Warning does not lead to the other symptoms or failures, lightmap baking is intact and works as expected.