"ArgumentNullException" errors are thrown and Project window freezes when renaming parent document while In Context mode

Steps to reproduce:

1. Create a new project
2. Create a new USS file (Create > UI Toolkit > Style Sheet (USS))
3. Create two UI Documents, name them "parent" and "child" (Create > UI Toolkit > UI Document (USS))
4. Open each Document in UI Builder
5. Click "+" > Add Existing USS and add previously created USS (on both Documents)
6. Open the "parent" in UI Builder and navigate to Project tab in Library tab
7. Expand the Assets foldout to find "child.uxml" and drag it into Hierarchy
8. Save the "parent.uxml" and drag it into the main Editor Hierarchy
9. From the Hierarchy, click on the arrow next to "parent.uxml" to open it In Context (Make sure UI Authoring is enabled in Project Settings)
10. In the opened context, press the arrow again next to "child.uxml"
11. From the Project window, rename the "parent.uxml"
12. Observe the Project window and Console

Actual results: "ArgumentNullException: Value cannot be null.
Parameter name: key" errors are thrown and the Project window freezes

Expected results: No errors and the Project window does not freeze

Reproducible with versions: 6000.7.0a3

Not reproducible with versions: 6000.6.0b5, 6000.7.0a2 (only warnings present)
Can't test with: 6000.5.5f1 (In Context editing not available)

Tested on (OS): macOS 26.5

Notes:

• Other windows remain functional. To recover Editor functionality after reproduction, navigate back from the "child.uxml"
• A separate console warning was also observed once - "SelectorAccelerationCache: ... has changed but change was not notified to SelectorAccelerationCache, please report a bug" - triggered by navigating back via breadcrumbs after reproduction. This did not reproduce consistently on retry