Steps to reproduce:

1. Create a URP 3D Project
2. Ensure you are using the advanced Object Picker:

Preferences > Search > Search Engines > Object Selector > Advanced

1. Create an Animator Controller and open it
2. Create an empty State and select it
3. In the Inspector for that State, try and pick a Motion with the circle to the right of the Motion field

Actual results: Warning "Unknown pseudo class "multiline"
UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)" logged everytime you pick a Motion, after picking it, the Warning will also log when entering Play Mode or importing the Asset

Expected results: No Warning is logged

Reproducible with versions: 6000.0.28f1, 6000.0.34f1, 6000.1.0b3

Not reproducible with versions: 2022.3.56f1, 6000.0.27f1

Tested on (OS): Windows 11