UI Builder window does not open on restarting the editor when it was open while restarting.

Steps to repro:
1. Create a new Project.
2. Window > UI Toolkit > UI Builder.
3. Close the Editor.
4. Open the same project.

Actual Result:
UI Builder window does not open on restarting the editor.

Expected Result:
UI Builder window opens up with the correct document.

Reproducible in:
2021.1.0a10, 2021.1.0a5.

Working Fine in:
2020.2.0f1.

Note: Unable to test in 2021.1.0a4 to 2021.1.0a1 due to some errors after installing UI Builder package.

Environment:
Occurring on Windows 10 & Mac 10.15.