Problem on patching permission groups

When @marc.hanisch wanted to add permission groups to an existing device, he run into an error:

{
  "errors":[
    {
      "detail":"'NoneType' object is not iterable",
      "status":"500",
      "title":"Unknown error"
    }
  ],
  "jsonapi":{
    "version":"1.0"
  }
}

The problem seem to be in the allow_only_admin_in_a_permission_group_to_remove_it_from_an_object function. It is called with None - which comes from the database instead of an empty list.