How to reproduce:
Note: do not delete your library folder at any point, this seems to fix the bug
1. Create a new project
2. Add Universal RP to the project (through the package manager or the manifest)
3. Create a Universal RP Pipeline Asset and use it in the graphics settings
4. Create a PBR shadergraph or Universal RP Lit shadergraph
5. Right-click the shadergraph and select Create -> Material
6. Click the material to make sure the inspector preview is not pink (if the bug is reproducing the preview in the inspector should render properly)

Expected result: the material thumbnail in the asset browser becomes correct
Actual result: the material thumbnail in the asset browser is pink

Reproducible with: 2020.2.0a18(10.0.0-preview.21), 2020.2.0a19(10.0.0-preview.21)
Not reproducible with: 2019.4.5f1(7.4.1), 2020.1.0f1(9.0.0-preview.34), 2020.2.0a17(10.0.0-preview.21)

Notes:
1. Happens with URP and HDRP. Did not reproduce with the Built-in pipeline
2. Tested on Windows10