Prerequisites: pull the repro project from here: https://ono.unity3d.com/test-data/gi-tests/LightmappedTrees

Steps to repro:

1. Open downloaded project and 'Terrain-medium' scene on a laptop with GeForce;
2. In the Lighting window switch Lightmapper to 'Progressive GPU';
3. In the Lighting window click "Generate Lighting" button;
4. When Lightmapper switches back to 'Progressive CPU', switch Lightmapper back to 'Progressive GPU';
5. Repeat step 4.

Expected result:
Lightmapper is switched to Progressive CPU and Editor doesn't crash or throw errors.

Actual result:
Editor crashes without BugReporter and with empty stacktrace. See attached video.

Notes:
- Reproducible in 2019.1.0a1, 2018.3.0b1;
- Reproducible in Windows 10;
- Reproducible only when baking on GeForce 640M with 2GB of VRAM; not reproducible on GeForce GTX 1070.