NullReferenceException is thrown when saving a document after changing a StyleSheet value and undoing with the same document open in two UI Builder windows

Steps to reproduce:

1. Create a new project
2. Open Edit > Project Settings > UI Toolkit and enable the "Enable Multiple UI Builder Windows" toggle
3. In the Project window, right-click > Create > UI Toolkit > UI Document; double-click the UXML to open it in UI Builder
4. Open a second UI Builder window: Window > UI Toolkit > UI Builder (New Window), and open the SAME UXML in it
5. Add a StyleSheet to the document (in the StyleSheets pane, "Add new USS" / assign a .uss) and add a selector then save
6. Select the selector in the StyleSheets list, then change a style value for it in the Inspector
7. Undo the change and Save

Actual results: 

A NullReferenceException is logged:

NullReferenceException: Object reference not set to an instance of an object
Unity.UI.Builder.StyleSheetExtensions.RemoveSelector (StyleSheet styleSheet, StyleComplexSelector selector, String undoMessage)
Unity.UI.Builder.StyleSheetUtilities.RemoveFakeSelector (VisualElement selectorElement)
Unity.UI.Builder.StyleSheetUtilities.OnSelectorElementDetachedFromPanel (DetachFromPanelEvent e)
... (event dispatch: DetachFromPanel) ...
UnityEngine.UIElements.VisualElement.Clear ()
Unity.UI.Builder.BuilderSharedStyles.ClearContainer (VisualElement documentElement)
Unity.UI.Builder.BuilderDocumentOpenUXML.ClearCanvasDocumentRootElement (...)
Unity.UI.Builder.BuilderDocumentOpenUXML.ReloadDocumentToCanvas (...)
Unity.UI.Builder.BuilderDocumentOpenUXML.ReloadDocument (...)
Unity.UI.Builder.BuilderDocumentOpenUXML.SaveNewDocument (...)
Unity.UI.Builder.BuilderDocument.SaveNewDocument (...)
Unity.UI.Builder.BuilderToolbar.SaveDocument (Boolean isSaveAs)
Unity.UI.Builder.Builder.SaveChanges ()
Unity.UI.Builder.Builder.<SaveOnKeyDownEvent>b__70_0 ()

Expected results: Saving after a change + undo does not throw. The stale/orphaned fake selector is either revalidated or skipped.

Reproducible with versions: 6000.7.0a6

Can’t test with versions: 6000.0.84f1, 6000.3.24f1, 6000.5.11f1, 6000.6.1f1 (No multiple UI Builder Windows feature)

Tested on (OS): Windows 11