Steps to reproduce:

1. Open a Unity Project
2. Open UI Builder via Window > UI Toolkit > UI Builder
3. Create a new StyleSheet in the StyleSheets  section via + > Create New USS. Give the file any name and Save it
4. Select the new StyleSheet and add a Selector to it in the Inspector
5. Expand the .uss file and select the created Selector.
6. At the bottom of the inspector, there should be a 'Text Property Preview' section
7. Click the More Option ( : ) menu and select 'Convert to Floating Window'
8. Dock the floating UI Builder Text Property Preview Window next to UI Builder

Actual results: The UI Builder Window breaks (becomes completely blank) and a NullReferenceException is logged to console

Expected results: The Text Property Preview Window can be docked without breaking The UI Builder Window and causing errors.

Reproducible with versions: 6000.1.0a4, 6000.0.26f1

Not reproducible with versions: - 

Can’t test with versions: 2022.3.52f1, 2021.3.45f1

Tested on (OS): Windows 10/11

Notes: 

• Can not test with pre Unity 6 versions as they do not have Text Property Preview feature