Erro quando imprimo o xml via java

Pessoal estou com uma árvore de classificação no formato xml, só que quando imprimo ela aparece o seguinte erro:

Warning: validation was turned on but an org.xml.sax.ErrorHandler was not
set, which is probably not what is desired. Parser will use a default
ErrorHandler to print the first 10 errors. Please call
the ‘setErrorHandler’ method to fix this.
Error: URI=file:D:/Projetos/OntoXML03/OntoXML01.xml Line=6: Document root element “knowledge_base”, must match DOCTYPE root “null”.
Error: URI=file:D:/Projetos/OntoXML03/OntoXML01.xml Line=6: Document is invalid: no grammar found.

Linhas iniciais, até a 6

<?xml version="1.0" ?>

<knowledge_base
	xmlns="http://protege.stanford.edu/xml"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://protege.stanford.edu/xml http://protege.stanford.edu/xml/schema/protege.xsd">

Não tenho idéia de como resolver.
Se alguem puder ajudar au agradeço.