How to reproduce:
1. Open the attached "UIToolkitBug" project
2. Go to Assets -> UI and open the "SettingsUI.uxml" Asset
3. Under the StyleSheets, expand the "OverrideDefault.uss"
4. Select the "unity-toggle_checkmark" Selector
5. In the Inspector, change the Background to any color
6. Close the UI Builder window and press "Discard" in the pop-up
7. Observe the results

Expected results: No errors are thrown
Actual results: Assertion and IndexOutOfRange errors are thrown

Reproducible with: 2022.3.59f1, 6000.0.41f1, 6000.1.0b9, 6000.2.0a6

Reproducible on: macOS Sequoia 15.3 (M1)
Not reproducible on: No other environment tested

Notes:
- After that, errors might be thrown on reopening the same Asset
- Not reproducible if changes are made when the UI Builder window opens together with the project. The Asset should be reopened
- After the errors, all UI in the scene is no longer rendered until you restart the proejct

Full errors: 
- Assertion failed
UnityEditor.EditorApplication:Internal_CallUpdateFunctions () (at /Users/bokken/build/output/unity/unity/Editor/Mono/EditorApplication.cs:384)
- IndexOutOfRangeException: Index was outside the bounds of the array.
UnityEngine.UIElements.StyleSheets.StylePropertyReader.SetCurrentProperty () (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/StyleSheets/StylePropertyReader.cs:652)