How to reproduce:
1. Open the user's attached "Atlas_Test" project
2. Open the Scene "SampleScene"
3. In the Hierarchy select the "Square" GameObject
4. In the Inspector, Sprite Renderer Component, press on the little circle of the Sprite property variable's name to open the selector window

Expected results: The selector window has a variety of sprites to select
Actual results: The selector is empty, only the "None" option is available

Reproducible with: 2021.2.0a1
Not reproducible with: 2018.4.30f1, 2019.4.18f1, 2020.1.17f1, 2020.2.2f1, 2021.0b2

Notes:
- You can still drag and drop Assets to the field to change it
- The issue is not reproducible with any of the Material selectors (maps, masks, etc.)
- The Material Select Object in the Sprite Renderer Component works

Workaround: Change Project Search engine settings to Legacy in the Preference window (Edit > Preference > Search > Search Engine > Project > Legacy)