[FIX] core: prevent KeyError when module is uninstalled

[reka-odoo]

When the user uninstalls the module and still tries to access the fields of
that module in settings, a KeyError will be generated.

Steps to reproduce the error:

• Go to settings > enable Use a Gmail Server
• Uninstall google_gmail
• Enter a value for ID > Discard

Traceback:

KeyError: 'google_gmail_client_identifier'
  File "odoo/http.py", line 2254, in __call__
    response = request._serve_db()
  File "odoo/http.py", line 1830, in _serve_db
    return self._transactioning(_serve_ir_http, readonly=ro)
  File "odoo/http.py", line 1850, in _transactioning
    return service_model.retrying(func, env=self.env)
  File "odoo/service/model.py", line 134, in retrying
    result = func()
  File "odoo/http.py", line 1828, in _serve_ir_http
    return self._serve_ir_http(rule, args)
  File "odoo/http.py", line 1835, in _serve_ir_http
    response = self.dispatcher.dispatch(rule.endpoint, args)
  File "odoo/http.py", line 2060, in dispatch
    result = self.request.registry['ir.http']._dispatch(endpoint)
  File "odoo/addons/base/models/ir_http.py", line 220, in _dispatch
    result = endpoint(**request.params)
  File "odoo/http.py", line 742, in route_wrapper
    result = endpoint(self, *args, **params_ok)
  File "addons/web/controllers/dataset.py", line 38, in call_kw
    return self._call_kw(model, method, args, kwargs)
  File "addons/web/controllers/dataset.py", line 34, in _call_kw
    return call_kw(request.env[model], method, args, kwargs)
  File "odoo/api.py", line 458, in call_kw
    result = getattr(recs, name)(*args, **kwargs)
  File "addons/web/models/models.py", line 871, in onchange
    defaults = self.default_get(missing_names)
  File "odoo/addons/base/models/res_config.py", line 464, in default_get
    classified = self._get_classified_fields(fields)
  File "odoo/addons/base/models/res_config.py", line 424, in _get_classified_fields
    field = self._fields[name]

sentry-4636111851

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