Steps to reproduce:
1. Create a new project in 2019.1
2. Install Addressable system 0.4.8 from the Package manager window
3. Upgrade project to 2019.2
4. The addressable system will throw an error in the Console
5. Reset packages Help -> Reset packages to default
6. Open Package Manager window

Results: Package manager window's sidebar is unresponsive. Closing and reopening window will fix this bug.
The bug is reproducible with all packages that throw an error after upgrading the project.

Reproduced with Package Manager UI: 2.1.1