title: Odoo 15, RPC_ERROR error message when updating module
body: <p>I created a custom module, and installed in my Odoo 15 session, after some changes, I tried to update it, but I get the following error message,</p>
  <p>How to fix it please?
  Thanks,</p>
  <pre>
  
      **RPC_ERROR**
      Odoo Server Error
      Traceback (most recent call last):
      File "C:\Program Files\Odoo15\server\odoo\addons\base\models\ir_http.py", line 237, in _dispatch
      result = request.dispatch()
      File "C:\Program Files\Odoo15\server\odoo\http.py", line 688, in dispatch
      result = self._call_function(**self.params)
      File "C:\Program Files\Odoo15\server\odoo\http.py", line 360, in _call_function
      return checked_call(self.db, *args, **kwargs)
      File "C:\Program Files\Odoo15\server\odoo\service\model.py", line 94, in wrapper
      return f(dbname, *args, **kwargs)
      File "C:\Program Files\Odoo15\server\odoo\http.py", line 349, in checked_call
      result = self.endpoint(*a, **kw)
      File "C:\Program Files\Odoo15\server\odoo\http.py", line 917, in __call__
      return self.method(*args, **kw)
      File "C:\Program Files\Odoo15\server\odoo\http.py", line 536, in response_wrap
      response = f(*args, **kw)
      File "C:\Program Files\Odoo15\server\odoo\addons\web\controllers\main.py", line 1339, in call_button
      action = self._call_kw(model, method, args, kwargs)
      File "C:\Program Files\Odoo15\server\odoo\addons\web\controllers\main.py", line 1327, in _call_kw
      return call_kw(request.env[model], method, args, kwargs)
      File "C:\Program Files\Odoo15\server\odoo\api.py", line 464, in call_kw
      result = _call_kw_multi(method, model, args, kwargs)
      File "C:\Program Files\Odoo15\server\odoo\api.py", line 451, in _call_kw_multi
      result = method(recs, *args, **kwargs)
      File "", line 2, in button_immediate_upgrade
      File "C:\Program Files\Odoo15\server\odoo\addons\base\models\ir_module.py", line 74, in check_and_log
      return method(self, *args, **kwargs)
      File "C:\Program Files\Odoo15\server\odoo\addons\base\models\ir_module.py", line 662, in button_immediate_upgrade
      return self._button_immediate_function(type(self).button_upgrade)
      File "C:\Program Files\Odoo15\server\odoo\addons\base\models\ir_module.py", line 600, in _button_immediate_function
      registry = modules.registry.Registry.new(self._cr.dbname, update_module=True)
      File "C:\Program Files\Odoo15\server\odoo\modules\registry.py", line 87, in new
      odoo.modules.load_modules(registry, force_demo, status, update_module)
      File "C:\Program Files\Odoo15\server\odoo\modules\loading.py", line 513, in load_modules
      env['ir.model.data']._process_end(processed_modules)
      File "C:\Program Files\Odoo15\server\odoo\addons\base\models\ir_model.py", line 2269, in _process_end
      self._process_end_unlink_record(record)
      File "C:\Program Files\Odoo15\server\odoo\addons\website\models\ir_model_data.py", line 36, in _process_end_unlink_record
      return super()._process_end_unlink_record(record)
      File "C:\Program Files\Odoo15\server\odoo\addons\base\models\ir_model.py", line 2198, in _process_end_unlink_record
      record.unlink()
      File "C:\Program Files\Odoo15\server\odoo\addons\base\models\ir_model.py", line 1375, in unlink
      self._process_ondelete()
      File "C:\Program Files\Odoo15\server\odoo\addons\base\models\ir_model.py", line 1423, in _process_ondelete
      ondelete = (field.ondelete or {}).get(selection.value)
      Exception
      
      The above exception was the direct cause of the following exception:
      Traceback (most recent call last):
      File "C:\Program Files\Odoo15\server\odoo\http.py", line 644, in _handle_exception
      return super(JsonRequest, self)._handle_exception(exception)
      File "C:\Program Files\Odoo15\server\odoo\http.py", line 302, in _handle_exception
      raise exception.with_traceback(None) from new_cause
      AttributeError: 'str' object has no attribute 'get'
  
  </pre>