ArgumentNullException error is thrown and Editor freezes when Editor property is set to Default.tss in Project Settings

Steps to reproduce:

1. Create a new Unity project using the Universal 3D template
2. Open the Project Settings window (Edit > Project Settings)
3. Open the UI Toolkit tab in the Project Settings window
4. Change the "Editor (for Editor Extension Authoring)" property to "Default.tss"
5. Observe the Console window
6. Close the Project Settings window
7. Repeat step 2
8. Observe the Editor

Actual results: “ArgumentNullException: Value cannot be null.” error is thrown and the Editor freezes when the Project Settings window is reopened

Expected results: No errors are thrown and the Editor does not freeze

Reproducible with versions: 6000.4.0a3

Can’t test with versions: 6000.0.60f1, 6000.2.8f1, 6000.3.0b7 (the feature is only in 6000.4.0a3)

Tested on (OS): MacOS Silicon Sequoia 15.6.1