NullReferenceException is thrown when instantiating a VisualElement created from a visualTreeAssetSource and there are multiple UI Documents in the Scene

How to reproduce:
1. Open the attached user project “HierarchyBug.zip“
2. Open the “SampleScene” Scene
3. Enter Play Mode

Expected result: No errors in the Console
Actual result: “NullReferenceException“ error in the Console

Reproducible with: 2022.1.0a6, 2022.3.1f1, 2023.1.0b21, 2023.2.0a18
Not reproducible with: 2021.3.27f1, 2022.1.0a5

Reproduced on: macOS 13.4 (Intel)

Note: This only happens when there are more than one UI Documents in the scene. Disabling the “Panel2“ GameObject will not result in a “NullReferenceException“ error in the Console