Behavior when creating a String ID entity is inconsistent between REST API and Generic UI

[vyacheslav-pushkin]

Environment

• Platform version: 7.2.1
• Addon version: 7.2.0

Description of the bug or enhancement

Attempting to create a String ID entity instance with an ID that already exists will result in a different behavior in Generic UI and REST API. In Generic UI this will result in an error message Unique constraint violation occurred. REST API will modify the existing entity. This results in inconsistent behavior between Generic UI and Frontend UI.