500 Servlet Exception

java.lang.NullPointerException
	at SirveObras.outFicheroAServir(SirveObras.java:211)
	at SirveObras.sirveFichero(SirveObras.java:645)
	at SirveObras.doGet(SirveObras.java:920)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:151)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:92)
	at com.caucho.server.dispatch.ServletFilterChain.doFilter(ServletFilterChain.java:106)
	at com.caucho.server.webapp.WebAppFilterChain.doFilter(WebAppFilterChain.java:178)
	at com.caucho.server.dispatch.ServletInvocation.service(ServletInvocation.java:229)
	at com.caucho.server.http.HttpRequest.handleRequest(HttpRequest.java:268)
	at com.caucho.server.port.TcpConnection.run(TcpConnection.java:389)
	at com.caucho.util.ThreadPool.runTasks(ThreadPool.java:507)
	at com.caucho.util.ThreadPool.run(ThreadPool.java:433)
	at java.lang.Thread.run(Thread.java:595)
Resin-3.0.19 (built Mon, 15 May 2006 04:50:47 PDT)