UXML object and its elements become no longer displayed in the Hierarchy after attempting to add an invalid element in UI Viewport window

Steps to reproduce:

1. Open the attached project
2. Open the SampleScene
3. Expand the PanelRenderer object and click on the > button in the Scene Hierarchy to open the UXML asset in context
4. Right-Click on any of the UI elements and select the Standard Elements -> UI Library... option
5. In the UI Library window, right-click on the MultiColumnListView element
6. Select the Add Element option
7. Repeat step 5
8. Select the Add Child Element option
9. Observe that "InvalidOperationException: You can't add directly to this VisualElement." errors are thrown
10. Press the < button to exit UI Viewport window
11. Observe the PanelRenderer object

Actual results: UXML asset and its elements are no longer displayed in the Hierarchy

Expected results: The UXML asset and its elements are displayed as before attempting to add an invalid node

Reproducible with versions: 6000.6.0b1, 6000.6.0b4, 6000.7.0a3

Can’t test with versions: 6000.0.79f1, 6000.3.20f1, 6000.5.4f1, 6000.6.0a7 (UI Viewport window not available)

Tested on (OS): macOS 26.5.2

Notes:

• The issue can be worked around by setting the PanelRenderer's source asset to None and then back to the original UXML asset