In the Root Index Settings folder to start indexing form when 'None' Asset is selected then an Argument exception is thrown in the console. Refer attached video and Stack Trace.

Steps to repro:
1. Create a new project
2. Window > Search > Search Index Manager
3. Add any Index Settings from Assets, Packages, Prefabs and Scenes
4. Now Root settings > Select Default Asset as 'None'.
(Toggle asset to None if any other is selected by default)

Actual Result:
Argument exception is thrown on selecting 'None' Asset for Roots index setting.

Expected Result:
No error should be in the console and None' Asset for Roots index setting selected gracefully.

Occurring on:
2021.2.0a6, 2021.2.0a1, 2021.1.0b8, 2021.1.0a1

Note:
Quick Search is a Built-In Package in the Unity 2021.1.0a1

Environment:
Occurring on Windows 10 & macOS 10.15.