docs: def load_app(self, app):
          """
          Tries to load an initial data class for a specified app. If the specified file does not exist,
          an error will be raised. If the class does exist, but it isn't a subclass of `BaseInitialData`
          then None will be returned.
          :param app: The name of the app in which to load the initial data class. This should be the same
              path as defined in settings.INSTALLED_APPS
          :type app: str
          :return: A subclass instance of BaseInitialData or None
          :rtype: BaseInitialData or None
          """
          if self.loaded_apps.get(app):
              return self.loaded_apps.get(app)
  
          self.loaded_apps[app] = None
          initial_data_class = import_string(self.get_class_path(app))
          if issubclass(initial_data_class, BaseInitialData):
              self.log('Loaded app {0}'.format(app))
              self.loaded_apps[app] = initial_data_class
  
          return self.loaded_apps[app]