Reproduction steps:
1. Open the attached “IN-114128.zip” project
2. From the Project window, open “BugReproduction.uxml” in UI Builder
3. Expand the test.uss dropdown
4. Click on the “.test-class”
5. Observe the Console

Expected result: No errors
Actual result: “Assertion failed”, “Trying to read value of type Enum while reading a value of type Keyword” and “ArgumentNullException: Value cannot be null.” errors are spammed

Reproducible with: 6000.2.0a8, 6000.2.3f1, 6000.3.0b1
Not reproducible with: 6000.0.56f1, 6000.2.0a7

Reproducible on: Windows 11
Not reproducible on: no other environments tested

Note:
- Sometimes UI Builder becomes blank after a lot of errors are spammed and UI Builder is opened