How to reproduce:
1. Open the attached project
2. Install HDRP (Window > Package Manager)
3. Open the "samplescene" Scene
4. Add "Custom Pass Volume" component to the camera
5. Drag the camera object to Project window

Actual result: Unity freezes or crashes.

Reproducible with: 2020.1.0a10 (HDRP 7.1.2), 2019.3.0b7 (HDRP 7.1.2).

Notes:
- Unity crashes on core::basic_string when trying to create prefab variant (freezes if creating original prefab).
- The issue is caused by "Custom Pass Volume" component.
- Custom Pass Volume component doesn't exist in 2019.2.10f1 (HDRP 6.9.1).