*Steps to reproduce:*
# Open a Unity Project
# Open UI Builder via Window > UI Toolkit > UI Builder
# Create a new StyleSheet in the StyleSheets  section via + > Create New USS. Give the file any name and Save it
# Select the new StyleSheet and add a Selector to it in the Inspector
# Expand the .uss file and select the created Selector.
# At the bottom of the inspector, there should be a 'Text Property Preview' section
# Click the More Option ( : ) menu and select 'Convert to Floating Window'
# 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