Steps to reproduce:

1. Create a new project using Universal 3D template
2. Go to Assets > Create > UI Toolkit > UI Document (UXML) and open it
3. Add a Visual Element to the Hierarchy and select it
4. In the Inspector, expand the Material foldout
5. Click on the object picker of the Material property
6. Observe the opened "Select Material" window
7. Select any item
8. Observe the Material property

Actual results: All built-in items in the object picker have no icons/previews. After selection, the property remains visually empty and throws the warning: Semantic - USS in 'style' attribute is invalid: Assets/NewUXMLTemplate.uxml (line 2): warning: Invalid URI location

Expected results: The object picker displays material icons and previews correctly, and selecting a material applies it to the visual element, updates the inspector field

Reproducible with versions: 6000.5.0b2, 6000.6.0a2

Not reproducible with versions: 6000.3.12f1, 6000.4.1f1, 6000.6.0a1

Tested on (OS): Windows 11

Notes:

• Perhaps the built-in Materials should not be shown at all due to them being incompatible