How to reproduce:
1. Open the attached project "texture.zip"
2. Select Assets/robot1.gif
3. Add a Secondary Texture to the list in the Sprite Editor window
4. Select Assets/robot2.jpg as the Secondary Texture
5. Click on the Secondary Texure in the list
6. Click on the empty space below the Secondary Texture
Expected results: Secondary Texture is unselected
Actual results: Secondary Texture keeps selected
Reproducible with: 2020.3.47f1, 2021.3.22f1, 2022.2.15f1, 2023.1.0b11, 2023.2.0a9
Reproducible on: Windows 10