Steps to reproduce:

1. Open any Project
2. Navigate to Edit > Project Settings > UI Toolkit
3. In the "Project Preview Theme" section open the dropdown menu for "Editor (for Editor Extension Authoring)" and switch to any of the options
4. Keep switching different themes for "Editor (for Editor Extension Authoring)" until the "Hold on" pop up appears

Actual results: Switching options for Editor (for Editor Extension Authoring) consecutively takes longer with each switch of the options, indicating a leak or no cleanup

Expected results: Switching Project Settings options does not take longer with each switch

Reproducible with versions: 6000.4.0a5, 6000.5.0a2

Not reproducible with versions: 6000.0.63f1, 6000.2.14f1, 6000.3.0f1 (No Project Settings for Project Preview Theme)

Tested on (OS): Windows 11

Note: Switching the options in Play Mode might hang the Editor for longer than 5 minutes