Prerequisites: For repro you'll need to download '[Particles] FTP' project from Collab.

Steps to repro:

1. Download the project from the comment section;
2. Open 't1' scene;
3. In the Lighting window click 'Generate Lighting' button;
4. Wait until the lighting bakes;
5. When the lighting bake is finished, save the scene;
6. Open 't2' scene;
7. Enter Play mode;
8. Exit Play mode;
9. Open 't1' scene;
10. In the Lighting window click 'Generate Lighting' button.

OR

1. Download and open '[Particles] FTP' project from Collab;
2. Editor will prompt to restart because the scripting backend has changed;
3. Click Restart button;
4. Open '18_LightsModule' scene;
5. Enter Play mode;
6. Wait until sprites are repacked;
7. Exit Play mode;
8. In the Lighting window click 'Generate Lighting' button.

Actual result:
Baking is stuck. See attached video.
Regression introduced in 2018.3.0b1.

Expected result:
Baking completes successfully.

Notes:
- Reproducible in 2019.1.0a4, 2018.3.0b5, 2018.3.0b1;
- Not reproducible in 2018.3.0a11, 2018.2.12f1, 2018.1.9f2;
- Reproducible on Windows and OSX;
- Reproducible when baking with GPU PLM and Enlighten; Not reproducible when baking with CPU PLM.
- If rebaking GI after clearing baked data after canceling stuck baking - all scene's lighting is black;
- The bug is not reproducible when not entering Play mode before rebaking GI;
- Restarting Editor fixes the issue.