Steps to repro:
1. Open the attached project and 'ProgressiveLightmappingExample' scene on OSX;
2. In the Lighting window click Generate Lighting button;

Expected result:
The lightbaking finishes without errors or crashes.

Actual result:
Editor crashes. See attached video. The stacktrace is meaningless.
Regression introduced in Version 2019.3.0a10.

Notes:
- Reproducible in versions 2019.3.16f1, 2019.3.0a10;
- Not reproducible in versions 2019.3.0a9, 2020.2.0a13, 2020.1.0b11, 2019.1.14f1, 2018.4.11f1;
- Reproducible only on OSX 10.15 and 10.14, not reproducible on Windows 10;
- Reproducible only with the Shadowmask mode, not reproducible in Subtractive and Baked Indirect;
- Reproducible in Metal and Open GLCore graphics API.