On adding/removing "new empty environment" in the "Look Dev" window throws ArgumentNullException when existing "New EnvironmentLibrary" is deleted, refer the attached video and Stacktrace.

Steps to repro:
1. Create a new project with HDRP Template
2. Upgrade High Definition RP Package to 7.1.7
3. Window > Render Pipeline > Look Dev
4. Select the "Environment" tab > New Library > Load Library
5. Add new empty environment (+)
6. Delete "New EnvironmentLibrary" from Project window
7. Add/Remove "new empty environment" under "Environment" section

Actual Result:
ArgumentNullException throws on adding/removing "new empty environment" under "Environment" section

Expected Result:
On deleting "New EnvironmentLibrary" from Project window, Library sets to None at runtime and existing "new empty environment" disappears

Note: "Look Dev" window introduced in HDRP Package 7.0.0

Tested with Unity version:
2020.1.0a19, 2019.3.0f6 (69ba09c9b97e)

Package found version: 7.1.7, 7.1.6, 7.1.5, 7.1.1, 7.0.0

Environment:
Windows 10 & macOS 10.14