How to reproduce:
1. Create a new project and open it
2. Go to Edit -> Project Settings -> Editor -> Version Control Mode dropdown, switch it to Perforce without entering any credentials
3. Open the Package manager and try to add the attached "LocalPackage" from disk
4. Observe the console errors

Expected result: no errors are spammed for the 2020.1.0a15+ versions and a simple warning/error is thrown describing why the manager can't add the package from disk (Because the version control is not properly connected yet and thus the manifest.json file is not writable. Something along the lines of: "Version control 'Perforce' is not connected; if you want to work without an active connection use 'Work Offline' option in editor settings")
Actual result: In the 2020.1.0a15+ versions a nullreference error is continuously spammed and on older than 2020.1.0a15 versions a nondescript error is thrown once

Reproduced in: 2020.1.0a15 (Nullreference errors are continuously spammed)
Not reproducible in: 2020.1.0a14, 2019.3.0f2, 2019.2.15f1, 2018.4.14f1 (No nullref spam but the error is undescript and could be improved)