Steps to reproduce:

Create a new 3D URP project
Go to Edit > Project Settings > Editor > Hierarchy
Enable "Use Legacy Hierarchy"
Right-click in the Hierarchy > UI Toolkit > Standard Elements > Visual Element
Observe the UI Viewport (in 6.6)
(Continue if 6.7+ version is used) save the .uxml asset when prompted
In the Scene view, with the UXML selected observe the UI Preview overlay appears
Click "Edit in UI Viewport"
Observe the UI Viewport

Actual results: The UI Toolkit editing stage opens while the legacy Hierarchy is used, and the Hierarchy doesn’t reflect it:

6000.6: the stage header gains an asterisk each time the UI Viewport is hovered until it’s filled, and the Hierarchy lists the main scene’s GameObjects
6000.7 / 7000: the Hierarchy window is empty

Expected results: The UI Toolkit editing stage can’t be entered while the legacy Hierarchy is used (the entry points, like Standard Element creation, are disabled or hidden) or a message explains that the new Hierarchy must be enabled

Reproducible with versions: 6000.6.6f1, 6000.7.0b4, 7000.0.0a8

Can’t test with versions: 6000.3.25f1 (Standard Elements not available for creation)

Tested on (OS): Windows 11