On settings "Scriptable Render Pipeline" Asset to "None" from the Graphics Settings, "HDRi Sky" Override couldn't add for "Volume Profile" Asset along with it NullReferenceExceptions are thrown, refer attached Stacktrace.docx and video.

Steps to repro:
1. Create a new project with HDRP Template
2. Upgrade High Definition RP Package to 7.1.8
3. Edit > Project Settings > Graphics > Scriptable Render Pipeline Settings > None
4. Assets > Create > Volume Profile
5. Select "New Volume Profile in the Project Window
6. Inspector > Add Override > Sky > HDRI Sky

Actual Result:
NullReferenceException is thrown on selecting "HDRI Sky" Override to "New Volume Profile" asset

Expected result :
"HDRi Sky" override option should be disabled if SRP Asset is not set with HD Render Pipeline Asset

Reproducible on:
2020.1.0a20, 2019.3.0f6, 2019.3.0b4 with package version 7.1.8, 7.1.7, 7.1.5

Working fine on:
2020.1.0a1, 2019.3.0b4 with package version 7.1.2

Package Found version: 7.1.7, 7.1.6, 7.1.5
Package Working fine version: 7.1.2, 7.1.1

Environment:
Windows 10 & macOS 10.14