This week's book giveaway is in the Mac OS forum. We're giving away four copies of a choice of "Take Control of Upgrading to Yosemite" or "Take Control of Automating Your Mac" and have Joe Kissell on-line! See this thread for details.
If you check the table of the API documentation org.w3c.dom Interface Node you will find that for an Element getNodeValue() will always return null. So either use the Node's getTextContent() method instead or append "/child::text()" to your XPath expression.