It should not be possible to add a Subagent with an Agent ID that is in use

Current Situation

• The Standalone Agent A1 is assigned Controller C1.
• The Cluster Agent CA1 is assigned C1
• Now it is possible to add a Subagent A1 to CA1 when A1 is defined with another URL.
• After that it is not possible to delete A1 from CA1.
• The following error is raised: BadRequestError: ItemIsStillReferenced: Item 'Subagent:A1' is still referenced by 'Agent:A1'

Desired Behavior

It should not be possible to add a Subagent with an Agent ID that is in use.