-NullReferenceException error is thrown on selecting installed cinemachine package.
-Unable to select installed cinemachine package to uninstall.
- Package Manager Window also turns blank on installing shader graph (8.0.1) package

Steps to repro:
1. Create a new project.
2. Windows > Package Manager > Cinemachine(2.6.0) > Install.
3. Select installed Cinemachine Package(2.6.0).

Actual Result:
Unable to uninstall Cinemachine Package.

Expected Result:
Able to uninstall Cinemachine Package.

Reproducible in:
2020.2.0a5.

Working Fine in:
2020.2.0a4, 2020.1.0b3.

Environment:
Occurring on Windows 10 & Mac 10.15.