On selecting static-dropdown in overrides window, 'ArgumentNullException' and 'NullReferenceException' errors thrown in console. Refer attached video and StackTrace.

Regression introduced in PR https://ono.unity3d.com/unity/unity/pull-request/113183/_/editor/replaceStaticAndLayerFieldsWithCustomDropdowns

Steps to reproduce:
1. Create a new project
2. Create two empty GameObjects "A" and "B" > Drag drop-in project to make prefabs
3. Make B a child of A.
4. Select A > In Override window select B > Open static-dropdown

Actual result:
ArgumentNullException errors are thrown on selecting static-dropdown in overrides window.

Expected result:
No errors in the console and static-dropdown should open gracefully.

Occuring in :
2021.1.0a8, 2021.1.0a2

Working fine in:
2021.1.0a1, 2020.2.0b14, 2020.1.12f1, 2019.4.14f1

Environment:
Windows10, macOS 10.15 and Ubuntu 18.04 LTS