Problem with recursive node

This work:

tipoDocumento = (String)wdContext.nodeDocumento().getTreeSelection().node().getChildNode("LineasFlujo", 0).getElementAt(0).getAttributeAsText("descripcion");

This crashes (nullpointer)

		tam = wdContext.nodeDocumento().getTreeSelection().node().getChildNode("LineasFlujo", 0).size();			

Why?

I want to access a subnode in a recursive node.

Recursive node - 0..n

each element of this recursive node has another node (a list). I'm using a treebynestingcolumn. When I click on an element, how I could get the element on the tree and his list inside?

Thanks in advance.