How to reprouduce:
1. Create a new Unity project
2. In the Project window, Create -> UI Toolkit -> TSS Theme File
3. Select the created TSS Theme File
4. In the Inspector, under the "Inherited Themes" click "+" -> Select the Asset
5. Observe the Assets for selection

Expected results: The list of available Assets doesn't allow to select the same Asset and create a circular import
Actual results: The list allows to select the same Asset and on "apply" this produces the warning and deletes the import

Reproducible with: 2021.3.46f1, 2022.3.53f1, 6000.0.28f1, 6000.1.0a5

Reproducible on: Windows 10