"ArgumentException: Object at index 0 is null" is thrown when clicking "Add Component" on a legacy UI Document Preset

Steps to reproduce:

1. Create a new 3D URP project
2. In the Hierarchy, right-click > UI Toolkit > Legacy > UI Document
3. In the Inspector, in UI Document Component, click on the preset icon (the sliders)
4. In the Preset window, click "Create New UI Document Preset..." and Save it
5. From the Project window, select the newly created preset
6. In the UI Document Component, click "Add Component"
7. Observe the Console

Actual results: "ArgumentException: Object at index 0 is null" errors and Component does not get added

Expected results: No errors - the button likely should not exist

Reproducible with versions: 6000.5.0b12, 6000.6.0a6

Can’t test with versions: 6000.3.17f1, 6000.4.10f1 (UI Document was not yet replaced by Panel Renderer)

Tested on (OS): Windows 11