Que erro é esse?

try {
			JREmptyDataSource ED = new JREmptyDataSource();
			JRBeanCollectionDataSource BCD = new JRBeanCollectionDataSource(variedades);
			ht.put("variedades", BCD);
			InputStream jp = session.getServletContext().getResourceAsStream("/Master.jasper");		

	               // O erro acontece nessa linha
			JasperReport relatorio = (JasperReport) JRLoader.loadObject(jp);

			JasperPrint impressao = new JasperPrint();
			impressao = JasperFillManager.fillReport(relatorio, ht, ED);
			byte[] buffer = JasperExportManager.exportReportToPdf(impressao);
			
			if (buffer != null){
				ServletOutputStream outputStream = response.getOutputStream();
				outputStream.write(buffer, 0, buffer.length);
				outputStream.flush();
				outputStream.close();
			}
			
		} catch (Exception e) {
			System.out.println("Erro ao gerar o relatório " + e.toString());
			e.printStackTrace();
		}

Erro ao gerar o relatório java.lang.NullPointerException

java.lang.NullPointerException

at java.io.ObjectInputStream$PeekInputStream.read(Unknown Source)

at java.io.ObjectInputStream$PeekInputStream.readFully(Unknown Source)

at java.io.ObjectInputStream$BlockDataInputStream.readShort(Unknown Source)

at java.io.ObjectInputStream.readStreamHeader(Unknown Source)

at java.io.ObjectInputStream.(Unknown Source)

at net.sf.jasperreports.engine.util.JRLoader.loadObject(JRLoader.java:191)

at br.com.fit.action.PrincipioAtivoAction.testeImpressao(PrincipioAtivoAction.java:412)

at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)

at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)

at java.lang.reflect.Method.invoke(Unknown Source)

at org.apache.struts.actions.DispatchAction.dispatchMethod(DispatchAction.java:274)

at org.apache.struts.actions.DispatchAction.execute(DispatchAction.java:194)

at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:419)

at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:224)

at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196)

at org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:414)

at javax.servlet.http.HttpServlet.service(HttpServlet.java:689)

at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)

at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)

at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)

at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)

at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)

at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)

at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)

at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869)

at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:667)

at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)

at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)

at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)

at java.lang.Thread.run(Unknown Source)[color=green][/color]