Steps to repro:

1. Open attached project and 'ProgressiveLightmappingExample' scene;
2. Go to the Lighting settings window (Window>Lighting>Settings);
3. Click 'Generate Lighting' button.

Expected result:
Scene starts to bake.

Actual result:
Scene is completely black and baking seems to not get started. See attached video.
Regression introduced in 2018.1.0a5.

Workaround is to click Generate Lighting button one more time - in this case lighting will start to bake.

Note:
- Reproducible in 2018.1.0a6, 2018.1.0a5;
- Not reproducible in 2018.1.0a4, 2017.3.0b11, 2017.2.0p3;
- Reproducible on both Windows and macOS;
- Reproducible only for PLM; not reproducible for Enlighten.