How to reproduce:
1. Open the project from the attached file "1230949.zip"(link in edit)
2. From Assets open "Lit Graph"
3. In the upper left corner of the Lit Graph window press Save Asset
4. Go to Scene View and inspect the Console

Expected behavior: no errors are thrown in the Console
Actual behavior: "MissingReferenceException: The object of type 'Material' has been destroyed but you are still trying to access it." error is thrown in the console

Reproducible with: ProBuilder 4.0.2(2019.3.9f1, 2020.1.0b4, 2020.2.0a5), ProBuilder 4.3.0-preview.6(2019.3.9f1, 2020.1.0b4, 2020.2.0a5)
Could not test with: 2017.4.38f1(no Package Manager), ProBuilder 3.10.1(2018.4.20f1)(Shader error in Console)

Notes:
- only reproducible when ProBuilder window is open in the project
- reproducible only once, you have to re-open the project to reproduce again.
- recompiling scripts stops throwing the error
- tested with these HDRP versions: 8.0.1, 7.3.1, 7.1.8
- Could not test with HDRP 4.10.0-preview(Shader error in Console)