Steps to reproduce:
1. Download repro project (link in the comments)
2. Open FAE_Demo scene
3. Navigate to the Lighting window
4. Select Progressive CPU as the lightmapping backend
5. Bake the lighting
6. Once the bake had finished, close the editor
7. Notice that it takes a long time for the editor to close
Notes:
- Reproduces in: 2019.4.14f1, 2020.1.17f1, 2020.2.0b14, 2021.1.0a2, 2021.1.0a9
- Reproduces when baking using CPU PLM; GPU PLM seems unaffected
- Does not reproduce with short bakes
- Editor can take over one minute to shutdown