Steps to reproduce:

1. Create a new URP project
2. Create and open a new UI Document
3. Add a "Scroll View" to the UI Builder Hierarchy (also reproduces with other available lists)
4. Create a new selector with a new USS file
5. Expand the "ScrollView" in the Hierarchy so that at least 2 child elements are visible
6. Drag the created selector between the greyed-out child elements
7. Observe the Console

Actual results: "ArgumentNullException: Value cannot be null. Parameter name: identifier" error is thrown 

Expected results: No errors are thrown

Reproducible with versions: 6000.4.0a2, 6000.4.5f1, 6000.5.0b6, 6000.6.0a4

Not reproducible with versions: 6000.3.14f1, 6000.4.0a1

Tested on (OS): Ubuntu 24.04, macOS 26.4

Notes:

• In 6000.4.0a1 and earlier, dragging selector would not show animation line indicating it can be placed inside Hierarchy and therefore, would not throw any errors
• After reproduction, doing anything in the UI Builder throws "ArgumentNullException: Value cannot be null."
• Reproduces with controls other than Scroll View that expose greyed-out built-in child elements in the Hierarchy