“+” button does nothing and has no functionality when clicked in Human Template "Bone Template" list

Steps to reproduce:

1. Create a new project using Universal 3D template
2. Import the attached "model.fbx"
3. Select the imported model
4. In the Inspector, navigate to the "Rig" tab
5. Switch the Animation Type to "Humanoid" and Avatar Definition to "Create From This Model"
6. Click "Apply" and then "Configure..."
7. Scroll to the bottom and click on "Mapping" dropdown
8. Click Save and save the Human Template asset in the Assets folder
9. In the Project window, select the newly created Human Template asset
10. In the Inspector, in the list section, click the "+" button
11. Observe the behavior

Actual results: clicking the "+" button does nothing and the list is not affected

Expected results: clicking the "+" button adds a new entry to the list (or otherwise performs a visible, meaningful action) or it is removed/disabled if it has no functionality

Reproducible with versions: 6000.0.67f1, 6000.3.7f1, 6000.4.0b7, 6000.5.0a6

Tested on (OS): Windows 11