Steps to repro:

1. Open attached project and 'ProgressiveLightmappingExample' scene;
2. In the Lighting window click Generate Lighting button;
3. Wait until the lighting is baked;
4. In the Lighting window go to 'Baked Lightmaps' tab;
5. Double-click on any lightmap in the Baked Lightmap tab of the Lighting window;
6. In the opened Lightmap preview window expand the dropdown list on the right side of Baked Lightmap label;
7. In the opened dropdown list choose 'Baked UV overlap' visualization.

Expected result:
Baked UV Overlap visualization is loaded and is shown in the lightmap preview window. See attached expected.png screenshot.

Actual result:
Baked UV Overlap visualization doesn't show the lightmap preview and is stuck on Loading. See attached actual.png screenshot.

Notes:
- Reproducible in 2019.1.0a3, 2018.3.0b5,
- Reproducible on Windows and OSX;
- Rebaking lighting fixed the issue.