Error 400 when trying to assign a user in a Test Execution that is not allowed to execute

Description
Error 400 when trying to assign a user in a Test Execution that is not allowed to execute

How to reproduce

1. In the Xray configuration, disallow executions for a given status,
2. Transition a Test Execution into to disallowed status,
3. Try to assign a user to a Test Run

Actual result/Attachments
Error message: _Unexpected error has occurred
Request failed with status code 400_

Expected result
Since the execution is not allowed, the option to assign a user should also not be allowed.