Steps to reproduce:

1. Create a new Unity project using a 3D Universal Template
2. Create a new UXML file and open it in UI Builder
3. In UI Builder create a new selector and a new USS file
4. Select the created selector
5. In the Inspector, expand the "Variables" foldout
6. Click on the "+" icon and select "AssetReference"
7. In the Texture2D field click on object picker
8. Select "Default-Particle" Texture 2D
9. Save the UXML file in UI Builder
10. Observe the Console and UI Builder

Actual results: UI Builder is not interactable and UXML assets corrupt and are not openable, "Assertion failed on expression: 'type != SubFilterType::Exact'" and "NullReferenceException: Object reference not set to an instance of an object" errors are thrown

Expected results: no errors or asset corruption

Reproducible with versions: 6000.3.5f1, 6000.4.0b6, 6000.5.0a5

Can’t test with versions: 6000.0.66f1 (Variables not available)

Tested on (OS): macOS 26.2 (M1)

Notes:

• Additionally, trying to close the Editor opens up Bug Reporter
• Trying to create new UXML files - creates ~5 files at once and none of them are openable