Assertion failed exception is thrown on selecting uXML file in Project window when it is opened & selected in UI Builder.

Steps to repro:
1. Create a new Project.
2. Window > UI Toolkit > UI Builder.
3. Viewport > File > Save > File name > TestuXML > Save.
4. Select TestuXML in Viewport > Select any other asset in Project Window.
5. Select TestuXML in the Project window.

Actual Result:
Assertion failed exception is thrown in the console.

Expected Result:
No error in the console.

Reproducible in:
2021.1.0a10, 2020.2.0f1, 2020.2.0b11.

Working Fine in:
2020.1.1f1

Environment:
Occurring on Windows 10 & Mac 10.15.

Note: Unable to test on 2020.2.b10 to 2020.2.0a1 due to errors on installing UI builder package.

Stack trace:
Assertion failed
UnityEditor.EditorApplication:Internal_CallUpdateFunctions ()