Steps to reproduce:

1. Open any Project
2. Navigate to UI Toolkit Project Settings and set "Editor (for Editor Extension Authoring)" to "Built-in Default Runtime Theme"
3. Create a UI Document Asset and open it in UI Builder
4. Select the root of the UI Document in the Hierarchy and enable "Editor Extension Authoring" checkbox flag
5. Observe the Console
6. In the UI Builder Window try opening the dropdown menu for the Runtime Theme
7. Close the UI Builder Window and open the Asset or any other UI Document Asset

Actual results: Error "NullReferenceException: Object reference not set to an instance of an object
UnityEditor.UIElements.ThemeUtility.NicifyThemeName" is logged in the Console, in some cases Users cannot open the Dropdown Menu for the Theme, UI Builder Window breaks, Editor has to be restarted

Expected results: Users are able to use the Built-in Theme

Reproducible with versions: 6000.4.0a5, 6000.5.0a2

Not reproducible with versions: 6000.0.63f1, 6000.2.14f1, 6000.3.0f1 (No Global Themes)

Tested on (OS): Windows 11, macOS Tahoe 26

Notes: UI Builder also breaks if the theme is set in UI Builder