Steps to repro:

1. Open attached project and ''Shadowmask' scene;
2. In the Hierarchy select ''Cube' gameobject;
3. In the Inspector under the Lightmaps>Realtime Lightmap foldout click 'Open Preview' button located on the lightmap preview thumbnail;
4. With the Realtime Preview window displayed open the Lighting Window;
5. In the Lighting window under Realtime Lighting foldout disable 'Realtime Global Illumination' checkbox;
6. Wait until lighting rebakes;
7. Observe the opened 'Realtime Lightmap for 'Cube'' window.

Expected result:
Lightmap preview is not shown. The 'Texture is currently not available' message is shown. See attached expected.png screenshot.

Actual result:
Lightmap preview is shown. Lightmap UVs are rendered for all gameobjects in the scene with overlapping. See attached video and actual.png screenshot.

Notes:
- Reproducible in 2019.1.0a1, 2018.3.0b2;
- Not reproducible in the previous versions, since the Lightmap preview window was introduced in 2018.3;
- Reproducible on OSX 10.13.6;
- The issue happens when disabling both Realtime and Baked GI.