In 'Tests Import Settings', at Assembly Definition References when duplicate reference GUI'd are used then on hitting play-mode to enter and exit causes editor to throw Errors. Refer attached video and stack trace.

How to reproduce:
1. Create a new project
2. Create Tests Assembly Folder
3. Project > Tests > Inspector
4. Assembly Definition References > Select any Duplicate assets.
5. Editor > Play mode > apply unapplied setting pop-up
6. Exit Play mode

Actual results: Errors are thrown on exiting editor Play-mode when the same reference GUI'd are used.

Expected results: no error in the console and related 'Assembly duplicate references' message should be thrown.

Occurring on:
2021.2.0a4, 2021.1.0b7, 2021.1.0b1

Working Fine:
2021.1.0a10, 2020.2.3f1

Environment:
Occurring on Windows 10 & macOS 10.15.