MultiColumnListView’s Sort Column Descriptions has a “+” button to add a list when the list is already added by default and does nothing when clicked

Steps to reproduce:

1. Create a new Unity project using the Universal 3D template
2. Open the UI Library (Right-click in the Hierarchy > UI Toolkit > Standard Elements > UI Library)
3. Double click the Multi Column List View to add it
4. Click Confirm
5. Click the "+" button on the right side of the Sort Column Descriptions in the Inspector window
6. Observe the Inspector window

Actual results: Nothing happens except the "+" button becomes a "-" button since the Sort Column Descriptions list is already added by default

Expected results: The "-" button is shown 

Reproducible with versions: 6000.6.0b3, 6000.7.0a2

Can’t test with versions: 6000.0.79f1, 6000.3.19f1, 6000.5.3f1 (due to no feature)

Tested on (OS): MacOS Silicon Tahoe 26.5.1, Ubuntu 24.04