The "Add selector..." option is missing from the UI Builders Hierarchy and Viewport options modal when options are not targeting an element

Steps to reproduce:

1. Create a new project
2. Right-click Project > Create > UI Toolkit > UI Document
3. Open it in UI Builder
4. Right-click in any empty space in the Hierarchy or an empty space in the Viewport, make sure to not target any Elements
5. Do it a second time
6. Observe the options menus bottom edge

Actual results: An options separation line is present at the very bottom of the options menu seemingly separating nothing

Expected results: Either "Add selector..." is reinstated and shown as greyed out option OR the separation line at the bottom of the options modal is removed

Reproducible with versions: 6000.0.9f1, 6000.0.42f1, 6000.1.0b10, 6000.2.0a6

Not reproducible with versions: 2022.3.59f1, 6000.0.8f1

Tested on (OS): Windows 11