Terrain Upgraded to SRP Materials are not saved when restarting the Editor
How to reproduce:
1. Open the attached "1332937_Repro" project
2. Open the Scene "SampleScene"
3. Convert all Materials to URP Materials (Edit->Render Pipeline->Universal Render Pipeline->Upgrade Project Materials to UniversalRP Materials)
4. Restart the Editor, reopen the "SampleScene"

Expected results: The Terrain has proper shading
Actual results: The Terrain is magenta in color

Reproducible with: 2020.2.0b10, 2020.3.7f1
Not reproducible with: 2019.4.25f1, 2020.1.17f1, 2021.1.0a4, 2021.1.6f1, 2021.2.0a16
Could not test with: 2018.4 because URP and HDRP were not yet available 2020.2.0a1, 2020.2.0b9, 2021.1.0a1, 2021.1.0a3 because Materials wouldn't convert (There's no upgrader to convert)

Notes:
- Reproducible with both Universal RP and High Definition RP packages
- SRP package versions don't have an impact on the issue