Steps to repro:

1. Open attached project and 'scene' scene;
2. In the Lighting window go to 'Baked Lightmaps' tab;
3. Double-click on the lightmap preview thumbnail to open the lightmap preview window.

Expected result:
Lightmap preview is rendered in the preview window. See attached expected.png screenshot.

Actual result:
Lightmap preview is not rendered in the preview window. See attached actual.png screenshot.
Regression introduced in 2018.3.0b1.

Workaround is to rebake lighting.

Notes:
- Reproducible in 2019.1.0a3, 2018.3.0b4, 2018.3.0b1;
- Not reproducible in 2018.3.0a11;
- Rebaking lighting fixes the issue;
- Not reproducible before 2018.3.