How to reproduce:
1. Open the attached 'TestPreviewRenderer2.zip' project
2. Navigate Example -> GameObject Editor
3. Observe that background is grey
4. Open the 'ExampleClass.cs' script
5. Observe that in 31 line 'OnPreviewGUI' method background image is set to a 'whiteTexture'

Expected result: ExampleClass window background color is white
Actual result: ExampleClass window background color is grey

Reproducible with: 2019.3.0a3, 2019.3.0f6, 2020.1.0a21
Not reproducible with: 2017.4.36f1, 2018.4.16f1, 2019.2.20f1, 2019.3.0a2

Notes:
1. Video of reproduction attached in Edit
2. Also reproducible with OnInteractivePreviewGUI method
3. Errors are thrown to the Console, when ExampleClass window is opened: 'RenderTexture.Create failed: width & height must be larger than 0'; 'ArgumentException: RenderTextureDesc width must be greater than zero.'
4. When opening 'Example Class' window for the first time it shows a blank window, reopening fixes the issue
5. Reproducible on macOS
6. Reproducible with linear and gamma color space