Reproduction steps:
1. Create a new project
2. Import Addressables package using Package Manager
3. Go Window -> Asset Management -> Addressables -> Groups
4. Dock the Addressables Groups window
5. Shut down the Editor and re-open the project

Expected result: Information in the Addressables Groups window will be visible
Actual result: The Addressables Groups windows is empty

6. Close the Addressables window and re-open it to resolve the issue

Reproducible with: 1.16.16 (2018.4.34f1, 2019.4.25f1, 2020.3.6f1, 2021.1.5f1, 2021.2.0a15), 1.16.19 (2020.3.6f1)
Not reproducible with: 1.16.15, 1.17.17 (2020.3.6f1)
Could not test with: 2021.2.0a15 because 1.16.16 package version is not available

Notes:
1. This is a package regression -> regression version: 1.16.16
2. The customer mentioned that the problem with the window results in an undeployable application (I was not able to verify this as we did not have access to the project)
3. The issue does not occur on any of the 1.17.x package versions