docs: def on_lock(self, widget, data=None):
          """Locks respective selected core element"""
          path_list = None
          if self.view is not None:
              model, path_list = self.tree_view.get_selection().get_selected_rows()
          models = [self.list_store[path][self.MODEL_STORAGE_ID] for path in path_list] if path_list else []
          if models:
              if len(models) > 1:
                  self._logger.warning("Please select only one element to be locked.")
              try:
                  self.model.global_variable_manager.lock_variable(models[0])
              except AttributeError as e:
                  self._logger.warning("The respective core element of {1}.list_store couldn't be locked. -> {0}"
                                    "".format(e, self.__class__.__name__))
              return True
          else:
              self._logger.warning("Please select an element to be locked.")