How to reproduce:
1. Open the attached “IN-136152” project
2. In Assets folder select the "TestAsset" asset
3. Drag and drop any sprite from the "Sprite" folder onto the "Atlased Sprite Ref" field in the "TestAsset" asset
4. Observe the Console

Actual result: An error “ArgumentNullException: Value cannot be null.” is thrown in the Console
Expected result: Editor rejects the invalid asset gracefully either by showing a controlled warning or refusing the assignment

Reproducible with: 1.28.2 (6000.0.70f1, 6000.5.0a8), 2.9.1 (6000.0.70f1, 6000.5.0a8)

Reproducible on: Windows 11
Not reproducible on: No other environments tested