code: private boolean checkGroupedColumn() {
          boolean allowEdit = true;
          if (getTreeTableRow().getTreeItem() != null) {
              Object rowObject = getTreeTableRow().getTreeItem().getValue();
              if (rowObject instanceof RecursiveTreeObject && rowObject.getClass() == RecursiveTreeObject.class) {
                  allowEdit = false;
              } else {
                  // check grouped columns in the tableview
                  if (getTableColumn() instanceof JFXTreeTableColumn && ((JFXTreeTableColumn) getTableColumn()).isGrouped()) {
                      // make sure that the object is a direct child to a group node
                      if (getTreeTableRow().getTreeItem().getParent() != null &&
                          getTreeTableRow().getTreeItem().getParent().getValue().getClass() == RecursiveTreeObject.class) {
                          allowEdit = false;
                      }
                  }
              }
          }
          return allowEdit;
      }
docs: only allows editing for items that are not grouped
  
  @return whether the item is grouped or not