text: Title: HTTP 500 on patch_group (KeyError)
  Body: **Steps to reproduce**
  _docker run -p 8888:8888 kinto/kinto-server_
  Running kinto 14.0.1.dev0.
  
  First request
  ```
  POST /v1/buckets HTTP/1.1
  Host: 127.0.0.1:8888
  Authorization: Basic AAAA
  Content-Type: application/json
  Content-Length: 222
  
  {"data": {"collection:schema": {}, "group:schema": {}, "record:schema": {}}, "permissions": {"collection:create": ["write_account"], "group:create": ["write_account"], "read": ["read_account"], "write": ["write_account"]}}
  ````
  
  First response HTTP 201
  ```
  {
      "permissions": {
          "read": [
              "read_account"
          ],
          "write": [
              "write_account",
              "account:admin"
          ],
          "collection:create": [
              "write_account"
          ],
          "group:create": [
              "write_account"
          ]
      },
      "data": {
          "collection:schema": {},
          "group:schema": {},
          "record:schema": {},
          "id": "6UULOFrV",
          "last_modified": 1608558398847
      }
  }
  ````
  Second request 
  ```
  PUT /v1/buckets/6UULOFrV/groups/0 HTTP/1.1
  Host: 127.0.0.1:8888
  User-Agent: python-requests/2.24.0
  Accept-Encoding: gzip,deflate
  Response-Behavior: diff
  If-None-Match: "0"
  Authorization: Basic AAAA
  Content-Type: application/json
  Content-Length: 83
  
  {"data": {}, "permissions": {"read": ["read_account"], "write": ["write_account"]}}
  ````
  Second response HTTP 201
  
  ```
  {
      "permissions": {
          "read": [
              "read_account"
          ],
          "write": [
              "write_account",
              "account:admin"
          ]
      },
      "data": {
          "members": [],
          "id": "0",
          "last_modified": 1608558411303
      }
  }
  ```
  
  Third request
  ```
  PATCH /v1/buckets/6UULOFrV/groups/0 HTTP/1.1
  Host: 127.0.0.1:8888
  User-Agent: python-requests/2.24.0
  Accept-Encoding: gzip,deflate
  Response-Behavior: diff
  If-None-Match: "0"
  Authorization: Basic AAAA
  Content-Type: application/json
  Content-Length: 83
  
  {"data": {}, "permissions": {"read": ["read_account"], "write": ["write_account"]}}
  ```
  
  Third response HTTP 500
  ```
  {
      "code": 500,
      "errno": 999,
      "error": "Internal Server Error",
      "message": "A programmatic error occured, developers have been informed.",
      "info": "https://github.com/Kinto/kinto/issues/"
  }
  ```
  
  Log
  ```
  "PATCH /v1/buckets/6UULOFrV/groups/0?" ? (? ms) 'id' errno=999
  
  Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/pyramid/tweens.py", line 41, in excview_tween
  response = handler(request)
  File "/app/kinto/core/events.py", line 159, in tween
  request.registry.notify(event)
  File "/usr/local/lib/python3.7/site-packages/pyramid/registry.py", line 109, in notify
  [_ for _ in self.subscribers(events, None)]
  File "/usr/local/lib/python3.7/site-packages/zope/interface/registry.py", line 448, in subscribers
  return self.adapters.subscribers(objects, provided)
  File "/usr/local/lib/python3.7/site-packages/zope/interface/adapter.py", line 619, in subscribers
  subscription(*objects)
  File "/usr/local/lib/python3.7/site-packages/pyramid/config/adapters.py", line 129, in subscriber_wrapper
  return derived_subscriber(*arg)
  File "/usr/local/lib/python3.7/site-packages/pyramid/config/adapters.py", line 101, in derived_subscriber
  return subscriber(arg[0])
  File "/app/kinto/views/groups.py", line 79, in on_groups_changed
  group_uri = f"/buckets/{event.payload['bucket_id']}/groups/{group['id']}"
  KeyError: 'id'
  "PATCH /v1/buckets/6UULOFrV/groups/0?" 500 (7 ms) agent=python-requests/2.24.0 authn_type=account errno=999 time=2020-12-21T12:47:57.573000 uid=admin
  ```