“None” Data Source Type option is missing when opening "Select Type…" in UI Builder Bindings

Steps to reproduce:

1. Create a new project using Universal 3D template
2. Go Assets > Create > UI Toolkit > UI Document (UXML)
3. Open the created .uxml asset
4. Add a Visual Element to the Hierarchy and select it
5. In the Inspector, under Bindings foldout, set "Data Source" to "Type"
6. Click the "Select Type..." button
7. Select any item
8. Click the "Select Type..." button
9. Observe the options

Actual results: No "None" item option is available for selection, making it impossible to set the "Data Source" Type back to None

Expected results: "None" is available for selection

Reproducible with versions: 6000.3.0a1, 6000.3.11f1, 6000.4.0f1, 6000.5.0a9, 6000.6.0a1

Not reproducible with versions: 6000.0.71f1, 6000.2.0b2

Tested on (OS): Windows 11