If you attempt to download multiple XR SDKs quickly, the following error will appear in the console:

Failed to resolve packages: An operation that requires exclusive access to the project is already running and must be completed before another can be started.. No packages loaded.

This causes all the previously loaded packages to disappear from the project until the next time the editor is booted.

Once you have started downloading a loader, we should probably stop the user accessing that loader context menu until the operation has completed.

1. Open Unity
2. Open the attached project
3. Go to Edit > Project Settings > XR Plugin Management
4. Download the Oculus Loader
5. As soon as you initiate the download, attempt to download the Oculus Loader again

Expected Result:
You cannot start another loader download until the previous one has completed.

Actual Result:
You can start another loader download. Package Manager loses exclusive access to the project manifest and no packages are loaded.

Tested on Windows 10.

Occurs on version 3.0.1 using 2019.3.0a11