Reproduction steps:
1. Create a new project
2. Right click the Project view and select Create > UI Toolkit > Editor Window
3. Let all 3 checkboxes marked, name your window, click Confirm
4. Open the created editor window, turn Live Reload on in for the window and dock it somewhere
5. Open the uxml created for the editor window in the UI Builder and edit some of its content (e.g. change the value of the label, or add a new visual element)

Expected result: the editor window live reloads, no errors are shown in the console
Actual result: the editor window does not live reload, there's a NullReferenceException shown on the console

Reproducible with - 2021.2.0a8, 2021.2.0a20
Not reproducible with - 2018.4.35f1, 2019.4.27f1, 2020.3.9f1, 2021.1.8f1, 2021.2.0a7