Reproduction steps:
1. Go to Unity Hub > Settings > General > Secondary Location Path
2. Type the invalid path directly in the input field and save
3. Back to Home > Installs, try to install any version of Unity.

Reproducible with: Unity Hub 2.0.2

Expected results: Some warning to notify the user that the path is invalid, maybe the disk was unmounted if the disk itself is not found, etc
Actual results: Nothing happens, the install doesn't start and we just see the same list of installs

Notes:
Also reproducible with mounting an external disk, selecting a directory on that disk, then removing the external disk.