Returns the NodeItem to use as the root for this tree layout.
the root node to use for this tree layout.
IllegalStateException if the action's data group does not
resolve to a
Explicitly sets the node to use as the layout root.
the node to use as the root. If the node is not a member of this layout's data group, an exception will be thrown.
IllegalArgumentException if the provided root is not a member of
this layout's data group.