NullReferenceException is thrown when you attempt to add any override to the Volume Profile asset.

Steps to repro:
1. Create a new project with HDRP Template using Unity 2020.3.0f1-dots
2. Upgrade High Definition RP Package to 10.3.2
3. Assets > Create > Volume Profile
4. Select "New Volume Profile" > Add Override > Exposure/Fog/Shadows (or any other override)

Actual Result:
NullReferenceException is thrown when attempting to add an override to the Volume Profile asset and the override is not added.

Expected Result:
Overrides should be added to "Volume Profile"

Note:
The issue does not seem to be present in 2020.2.4f1-dots with HDRP version 10.3.2
You can change the values on the overrides and delete them in the Volume Profile.