How to reproduce:
1. Open the attached project "Example_Rig_Bug.zip"
2. From the Project window, select Assets/Example_Rig_Bug.psb
3. In the Inspector window enter Sprite Editor
4. Enable the Visibility tool (Skinning Editor>Visibility tool)
5. In the Visibility tool select Sprite
6. In the Category column press on any arrow and Remove Empty Category

Expected result: Empty Category "Sprite_C" is removed
Actual result: Empty Category "Sprite_C" is not removed

Reproducible with: 3.2.6 (2019.4.24f1), 5.0.4 (2020.3.3f1, 2021.1.2f1, 2021.2.0a6)
Could not test with: 2.0.0-preview.3 (2018.4.34f1), 5.0.4 (2021.2.0a12), 6.0.0-pre.1 (2021.1.2f1, 2021.2.0a12), 6.0.0-pre.2 (2021.1.2f1, 2021.2.0a12), 6.0.1 (2021.1.3f1, 2021.2.0a15) (Category column not available)

Notes:
- If you do not change the Sprite Layer name, Remove Empty Category works as expected
- Could not reproduce with 5.0.4 (2021.2.0a12) because of an error