Steps to reproduce:

1. Open any Project (Reproduction was with HDRP) & Import the attached Unity Asset Package
2. Open Scene "Enemy Plates" and select "EnemyNamePlate" in the Hierarchy
3. Open the UI Document that's assigned to it
4. Enter Play Mode
5. Select the bottom Style Selector and change Text Color to any other Color
6. Close the UI Builder Window and Save
7. Open the UI Document again and observe the Console and UI Builder Window

Actual results: Errors "Assertion failed
UnityEngine.UIElements.VisualTreeStyleUpdaterTraversal:TraverseRecursive (UnityEngine.UIElements.VisualElement,int)
Unity.UI.Builder.BuilderVisualTreeStyleUpdaterTraversal:TraverseRecursive "
&
"NullReferenceException: Object reference not set to an instance of an object
UnityEngine.UIElements.VisualTreeStyleUpdaterTraversal.ProcessMatchedRules (UnityEngine.UIElements.VisualElement element, System.Collections.Generic.List`1[T] matchingSelectors) (at <652ff8c3d9f84f3faf38e12baa95e383>:0)"

are logged, the UI Builder for that Asset Brakes, resetting layout via right click fixes the issue after a couple tries, but opening it again produces same issues. Workaround is to create a new document or open a different document.

Expected results: UI Builder handles edits in Play Mode gracefully

Reproducible with versions: 6000.3.0a5, 6000.3.0b5, 6000.4.0a2

Not reproducible with versions: 6000.0.59f1, 6000.1.17f1, 6000.2.8f1, 6000.3.0a4

Tested on (OS): Windows 11