This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
The code you posted does not tell you that the "doc" variable contains null. It is possible that its toString() method returns null, or the 4-character string "null", as well. And considering that the getNodeValue() method of an Element node does return null (check the API documentation for org.w3c.dom.Node), it's possible you have misidentified the problem.