Steps to reproduce:

1) Open attached project (HDRP template)
2) Load the sample scene
3) Make sure that lightmapper is set to Progressive GPU
4) Make sure Spotlight has Distance Shadowmask mode and Shadowmask is the active mode in Lighting tab
5) Bake lighting
6) Change Distance Shadowmask to Shadowmask on the Spotlight
7) Clear and bake
8) Change back to Distance Shadowmask
9) Clear and bake

Result: Baking never finishes and when you try to quit Unity, it will get stuck.

Notes:

- Works in: 2019.3.4f1 with HDRP 7.1.3
- Does not work in: 2020.2.0a8 with HDRP 8.1.0
- The repro steps aren't very stable. Sometimes it reproduces on the second bake, sometimes you need to do it a few times (see attached video)
- If the issue does not reproduce, restart the editor and try again
- Tested on macOS; does not reproduce on Windows