"Accessing invalid property" error is thrown when refocusing UI Builder after saving an externally edited USS file

Steps to reproduce:

1. Create a new project using 3D URP template
2. Create a new UI Document (Assets > Create > UI Toolkit > UI Document)
3. Open the .uxml file in UI Builder
4. Create a new USS file with a new selector (e.g. "test")
5. Select the newly created selector
6. In the Inspector "Variables" foldout click "+" and add any variable (e.g. float")
7. Open the uss file in any text or code editor
8. Remove the line that declared the variable (e.g. "--new-var-1: 0;")
9. Save the uss file
10. Without closing the text editor, click on the Inspector in UI Builder
11. Observe the Console

Actual results: "Accessing invalid property" error is thrown

Expected results: No errors

Reproducible with versions: 6000.0.72f1, 6000.3.13f1, 6000.4.3f1, 6000.5.0b4, 6000.6.0a3

Tested on (OS): macOS 26.4, Ubuntu 24.04

Notes:

• Sometimes reproduces when adding code lines in the uss file as well
• Affects not only "Variables" and reproduces when editing most code lines
• Does not seem to reproduce if the text editor is closed before refocusing