Steps to reproduce:

Create a new Unity project using the Universal 3D template
Open the UI Library (Right-click in the Hierarchy > UI Toolkit > Standard Elements > UI Library)
Double click the Multi Column List View to add it and save the .uxml
In the Inspector, find the Columns > Columns foldout
In the field next to it, change the value from "0" to "2" and click enter
Observe two elements are created but are not expandable (and error is thrown)
Save the .uxml
Exit in-scene editing and observe

Actual results: "ArgumentException: Cannot insert null column" and "Invalid UXML or USS: System.ArgumentException: Cannot insert null column" errors when trying to reopen .uxml, the .uxml asset becomes corrupted and not-editable

Expected results: Two Columns are created with default values, no errors are thrown, and the .uxml remains editable

Reproducible with versions: 6000.6.0b1, 6000.6.0b7, 6000.7.0a4

Can’t test with versions: 6000.3.21f1, 6000.5.7f1 (in-scene UI Authoring not available)

Tested on (OS): Windows 11

Notes:

The corruption is not recoverable through the Editor UI. Undo does not restore the asset (sometimes even silently crashes editor), the MultiColumnListView is removed from the document, and the in-scene editing entry point no longer appears on the asset
To test on 6000.7.0a4, you must first open the .uxml in UI Builder due to ~~UUM-148758~~
The equivalent array size field is not exposed in the UI Builder window for the same Columns list