Cluster kubernetes API throws 500 error if user tries to submit another cluster

Summary

Cluster kubernetes API throws 500 if a user tries to submit multiple clusters on CE

Steps to reproduce

1. Add a cluster through an API or through UI
2. Add another cluster through API

❯ curl --header 'Private-Token: '$MAINTANER_PAT'' http://localhost:3000/api/v4/projects/131/clusters/user \
-H "Accept: application/json" \
-H "Content-Type:application/json" \
-X POST --data '{"name":"cluster-1", "platform_kubernetes_attributes":{"api_url":"https://35.111.51.20","token":"12345","namespace":"cluster-1-namespace"}}'

{"message":"500 Internal Server Error"}

This only happens on CE, as we only allow on cluster per project. On EE its not present.

What is the current bug behavior?

API throws 500 internal error server

What is the expected correct behavior?

API should return an appropriate error message

Relevant logs and/or screenshots

Output of checks

POST_error

This happens on gitlab.com

Possible fixes

Add a policy check on "POST" endpoint to check if the user can add another cluster.