Delete a Custom Field with a value returns a 500 server error causing an error to be displayed in the Test "Cannot read property 'id' of null"

Short Description: Delete a Custom Field with a value returns a 500 server error causing an error to be displayed in the Test "Cannot read property 'id' of null".

Steps to Reproduce:
1 - Create a Test with custom fields.
2 - Go to the Xray settings and delete the custom fields.

Actual Results: An error is displayed.
Expected Results: No error should be displayed and the custom field should be deleted.