Vaadin: how to set drop criterion on TreeGrid

Question

I'am trying to allow dropping items in a TreeGrid when a condition becomes true e.g. node is not a leaf.

Node:

public class Node {
    List<Node> children;
   ...
    public boolean isLeaf() {
      return children.isEmpty()
    }
}

TreeView:

public class TreeView extends TreeGrid<Node> {
    public TreeView(){
        ...
    TreeGridDropTarget<Node> dropTarget = new TreeGridDropTarget<>(this, DropMode.ON_TOP);
    dropTarget.setDropEffect(DropEffect.MOVE);
    dropTarget.addGridDropListener(event -> {
        event.getDragSourceExtension().ifPresent(source -> {
            Notification.show("it works");
            });
        });
     }
  }

How to check if grid element is allowed to contain subelement? So how to add acceptance criteria?