Steps to reproduce:

Create a 3D URP Project
Navigate to Project Settings > UI Toolkit and enable "Enable in-scene UI Authoring"
In the Project Browser create 2 UI Toolkit > UI Documents
Drag one into a Scene's Hierarchy and start editing it in Context by clicking the arrow button in the Hierarchy next to it
In it's Hierarchy drag and drop the other UI Document and drill into it with the same arrow button
When prompted to save select "Discard"

Actual results: On Discard the Console is flooded with errors and the in-Scene authoring view breaks:

ArgumentException: Provided TemplateAsset is not part of the 'Parent' VisualTreeAsset. Parameter name: subDocumentPath
NullReferenceException (repeating on repaint) via VisualElementSelectionRegistry.GetEditFlags
NullReferenceException (repeating on pointer move) via PanelElement.LocalToPanelPosition
The stage falls back to the Main Stage in a half-torn-down state and the two NREs keep firing continuously. Returning to the Main Stage does not stop them; only a reload of the Editor will enable in-scene Authoring again

Expected results: Clicking Discard reverts the nested-instance addition cleanly and returns to the parent document (or Main Stage) with no exceptions and a working authoring view

Reproducible with versions: 6000.6.0b8, 6000.7.0a4

Can’t test with versions: 6000.0.81f1, 6000.3.22f1, 6000.5.8f1 (No in-Scene UI Authoring)

Tested on (OS): Windows 11