message: cancel regroup operation if there is no drop target
diff: @@ -80,10 +80,16 @@ const nodeDragSourceSpec = (
  : undefined,
  canCancel: (monitor) => monitor.getOperation() === REGROUP_OPERATION,
  end: async (dropResult, monitor, props) => {
  + if (monitor.getOperation() === REGROUP_OPERATION) {
  if (monitor.didDrop() && dropResult && props && props.element.getParent() !== dropResult) {
  await moveNodeToGroup(props.element, isNode(dropResult) ? dropResult : null);
  dropResult.appendChild(props.element);
  + } else {
  + // cancel operation
  + return Promise.reject();
  }
  + }
  + return undefined;
  },
  collect: (monitor) => ({
  dragging: monitor.isDragging(),
  