Shader preview images are missing in Shader Asset selector window of Shader Variant Collection.

Steps to repro:
1. Create a new HDRP template project.
2. Project Window > Click on "+" > Shader > Shader Variant Collection.
3. Inspector > Click on Add Shader.

Actual Result:
Shader preview images are missing in Shader Asset selector window.

Expected Result:
Shader preview images showing in Shader Asset selector window

Reproducible in:
2020.2.0a14, 2020.1.0a12, 2020.1.0a23 with Package Version 9.0.0, 8.1.0, 8.0.1.

Environment:
Occurring on Windows 10 & Mac 10.15.