TwoPaneSplitView needs exactly 2 children error is thrown if TwoPaneSplitView element is selected in the UI Library window while Inspector tab is enabled

Steps to reproduce:

1. Open the attached project
2. Open the UI Library window (Window -> UI Toolkit -> UI Library)
3. Enable the Inspector tab in the UI Library window
4. Select the Two Pane Split View element
5. Observe the Console window

Actual results: "TwoPaneSplitView needs exactly 2 children." error is thrown each time the element is selected

Expected results: No warning or errors are thrown when selecting elements in the UI Library window

Reproducible with versions: 6000.6.0a7, 6000.6.0b7, 6000.7.0a4

Can’t test with versions: 6000.6.0a6 or below (UI Library window not accessible)

Tested on (OS): macOS 26.5.2