NullReferenceException error is thrown on creating new Lightmap Parameter in Lighting Settings.

Steps to repro:
1. Create a new project.
2. Windows > Rendering > Lighting.
3. Click on "New Lighting Settings" button.
4. Select New Lighting Settings > Lightmapping Settings.
5. Lightmap Parameters > Click on drop-down < Select Create New.

Actual Result:
NullReferenceException error is thrown in the console.

Expected Result:
No errors in the console.

Reproducible in:
2020.2.0a13, 2020.2.0a4.

Working Fine in:
2020.2.0a3, 2020.1.0b11.

Environment:
Occurring on Windows 10 & Mac 10.15.

Note: Please refer attached stack trace.