Deleting USS while it is opened in UI Builder corrupts the UXML file

Steps to reproduce:

1. Create and open a new project
2. Create a UXML and a USS file
3. Open the UXML file in UI Builder and add the USS file to 'StyleSheets' section
4. While UI Builder is open, delete the USS file from the Project window
5. Close the Dialog box and reopen the UXML file in UI Builder
6. Observe the UI Builder

Actual results: UXML gets corrupted "<unsaved file>" is displayed instead of the proper UXML name

Expected results: UXML loads up okay, the missing USS is handled, and the reference is removed

Reproducible with versions: 6000.0.0f1, 6000.0.77f1, 6000.3.18f1, 6000.4.12f1, 6000.5.0f1, 6000.6.0a7

Tested on (OS): Windows 11 Pro (25H2)

Notes:

• If the UXML is tried to be saved after deleting the USS, ArgumentException or MissingReferenceException will be thrown