"NullReferenceException" is thrown when opening the Source Asset object picker for Panel Renderer in Debug Inspector

Steps to reproduce:

1. Create a new 3D URP project
2. In the Hierarchy, right-click > UI Toolkit > Panel Renderer
3. Select the Panel Renderer GameObject 
4. Switch the Inspector mode from "Normal" to "Debug" (three dots next to Inspector tab > Debug)
5. In the Panel Renderer Component find the "Source Asset" property
6. Click on the object picker for the Source Asset
7. Observe the behavior

Actual results: "NullReferenceException" is thrown and the object picker does not open

Expected results: Object picker opens with no errors

Reproducible with versions: 6000.5.0a6, 6000.5.0b9, 6000.6.0a5

Can’t test with versions: 6000.4.8f1 (Panel Renderer Component not available)

Tested on (OS): Windows 11

Notes:

• Does not reproduce with Normal Inspector mode