code: protected function createModelIdentity(
          $modelClass,
          $id,
          $keyName = null
      ) {
          if (is_null($id)) {
              return null;
          }
  
          $model = new $modelClass();
  
          if (!$model instanceof Model) {
              throw new RuntimeException(sprintf('Expecting a model class, got %s.', $modelClass));
          }
  
          $model->setAttribute($keyName ?: $model->getRouteKeyName(), $id);
  
          return $model;
      }
docs: Create a model identity using the model class and a provided id.
  
  @param $modelClass
  @param string|int|null $id
  @param string|null $keyName
  the key to set as the id - defaults to `Model::getRouteKeyName()`
  @return Model|null
  @deprecated