How to reproduce:
1. Create a new project and open it
2. Add the localization package to the project (Add this line to the package manifest file - "com.unity.localization": "0.9.0-preview" )
3. Open the localization tab (Project settings -> Localization)
4. Select any of the "Locale selector" fields and right click -> duplicate array element
5. Observe that the field did not duplicate and an error is thrown instead

Expected result: array element is duplicated
Actual result: array element is not duplicated and an error is thrown instead

Reproduced in: 2021.1.0a5, 2021.1.0a1
Not reproducible in: 2020.2.0b10, 2020.1.12f1, 2019.4.14f1