code: public void deselectAll() {
          getDispatcher().beforeDeselect(this);
          
          new Select(getElement()).deselectAll();
          if (Config.getBoolConfigProperty(ConfigProperty.ENABLE_GUI_LOGGING)) {
              logUIActions(UIActions.CLEARED, "all");
          }
          
          getDispatcher().afterDeselect(this);
      }
docs: Clear all selected entries. This is only valid when the SELECT supports multiple selections.
  
  @throws UnsupportedOperationException
  If the SELECT does not support multiple selections