How to reproduce:
1. Create a new Unity project
2. Open the Asset Store and find an Asset that wouldn't be downloaded instantly
3. Open the Package Manager (Window -> Package Manager) and go to "My Assets"
4. Press download and unfocus the Editor

Expected results: The asset downloads normally
Actual results: After downloading the asset a pop-up appears and an error is thrown in the Console

Reproducible with: 2019.3.0b4, 2019.4.23f1, 2020.1.0a6
Not reproducible with: 2018.4.33f1, 2019.3.0b3, 2020.1.0a7, 2020.3.2f1, 2021.1.1f1, 2021.2.0a10

Note:
-In version 2020.1.0a6 there's a different result: an error is thrown and you are prompted to update the asset even though it is downloaded
-In version 2020.1.0a3 assets need to be downloaded from the Asset Store window and not from the Package Manager
- Regression in version 2019.3.0b4 was probably introduced because assets became downloadable from the Package Manager "My Assets" tab