Unable to delete NewAssembly from Assembly Definition References list in Test Import Setting. User can't select None option also

Steps to Reproduce:

1. Create a new Project
2. Open Windows > General > TestRunner
3. Select playmode in TestRunner Window
4. Create Assets > right-click > Assembly Definition
5. Click on Assets > Tests > Tests > Inspector > Assembly Definition References > "+"
6. Select Assets > Tests > Tests and drag Assets > NewAssembly > Assembly Definition Reference List
7. Delete Assets > Tests > Tests > Inspector > Assembly Definition References > New Assembly

Expected Result:
NewAssembly is deleted from the List

Actual Result:
NewAssembly is not deleted

Reproducible on: 2020.1.0a22, 2020.1.0a1, 2019.3.0f6, 2019.3.0a1, 2019.2.0a7

Working fine on: 2019.2.0a6, 2019.1.14f1, 2018.4.17f1

Work Around: User needs to remove the Key by clicking on "-" button

Environment: Windows 10 and Mac 10.14