[UI] The icon preview of the Lighting Setting Asset is missed

Steps to repro:
1. Open attached project and 'ProgressiveLightmappingExample' scene;
2. In the Project folder navigate to Assets>Scenes and select 'ProgressiveLightmappingExampleSettings' file.

Expected result:
The 'ProgressiveLightmappingExampleSettings' has the preview icon in both Project folder and Inspector. See attached screenshot on how the icons should look like.

Actual result:
There is no icon preview for the lighting settings asset file. See the attached screenshot.

Notes:
- The issue will be introduced with this branch: lighting/lighting-settings-asset-take2;
- Reproducible in Version 2020.1.0a12.