The UXML document hierarchy arrow button disappears when adding a UXML document via the UI Library

Steps to reproduce:

1. Create a new Unity project using the Universal 3D template
2. Create a new UXML document in the Project window (Right-click > Create > UI Toolkit > UI Document (UXML))
3. Add the UXML in the Hierarchy (Right-click > UI Toolkit > Standard Elements > UI Library)
4. Select the UXML document in the Hierarchy so it's highlighted
5. Click the arrow button next to the UXML document in the Hierarchy
6. Observe the Hierarchy

Actual results: Nothing happens and the arrow button disappears

Expected results: The UXML document is opened

Reproducible with versions: 6000.6.0b5, 6000.7.0a3

Can’t test with versions: 6000.0.80f1, 6000.3.20f1, 6000.5.5f1 (due to feature missing)

Tested on (OS): MacOS Silicon Tahoe 26.5.1

Notes:

• Reproduces only for the first time