Reproduction steps:
1. Open attached project "1293762.zip" and go to Assets folder
2. In the "Unos" folder create Assembly Definition named "Unos"
3. Select "Unos" Assembly definition and look at Inspector window
4. Lock Inspector window
5. Go to "Dos" folder and create Assembly Definition named "Dos"
6. In the Inspector window, under Assembly Definition References add a new Reference
7. Drag "Dos" Assembly definition to newly created Reference
8. Press Apply and unlock the Inspector window
9. Select "Dos" Assembly definition and lock Inspector window
10. Go to "Unos" folder
11. In the Inspector window, under Assembly Definition References add a new Reference
12. Drag "Unos" Assembly definition to newly created Reference
13. Press Apply

Expected result: Error "Assembly with cyclic references detected" is logged to console
Actual result: Exception "Exception: No pending assemblies queued for compilation and no compilers running. Compilation will never finish." is logged to console

Reproducible with: 2020.2.0b11, 2020.2.0f1
Not reproducible with: 2018.4.30f1, 2019.4.16f1, 2020.1.17f1, 2020.2.0b10, 2021.1.0a9