How to reproduce:
1. Open the ‘Tester.zip' project
2. Open the ‘MyUXML.uxml’ in the Project window
3. Select ‘Broken’ in the UI Builder hierarchy
4. In the UI Builder inspector scroll down to ‘Transition Animations’
5. Try to remove the field containing 'background-for 2s Ease 0s’
6. Observe the console

Expected result: No errors are thrown and the transition can be removed
Actual result: Errors are thrown and the transition cannot be removed

Reproducible with: 2023.1.0a1, 6000.0.62f1, 6000.2.12f1, 6000.3.0b9, 6000.4.0a4, 6000.5.0a1

Reproducible on: macOS 26.0.1 (M1 Max)
Not reproducible on: no other environments tested

Note: First line of error - ArgumentOutOfRangeException: Specified argument was out of the range of valid values.