Trying to download an empty, unopened project causes an error

[matthewhorridge]

Related to issue #33

Steps to reproduce:

1. Create an empty project. Do not open it.
2. Try to download the project. Observer that the download fails.

An exception is thrown on the server:

java.lang.NullPointerException: revisionRootOntology must not be null at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:208) at edu.stanford.bmir.protege.web.server.owlapi.OWLAPIProjectDocumentStore.applyRevisionMetadataAnnotationsToOntology(OWLAPIProjectDocumentStore.java:449) at edu.stanford.bmir.protege.web.server.owlapi.OWLAPIProjectDocumentStore.exportProjectRevision(OWLAPIProjectDocumentStore.java:188) at edu.stanford.bmir.protege.web.server.owlapi.OWLAPIProjectDocumentStore.createDownloadCacheIfNecessary(OWLAPIProjectDocumentStore.java:437) at edu.stanford.bmir.protege.web.server.owlapi.OWLAPIProjectDocumentStore.exportProject(OWLAPIProjectDocumentStore.java:160) at edu.stanford.bmir.protege.web.server.filedownload.OWLAPIProjectDownloader.writeProject(OWLAPIProjectDownloader.java:51) at edu.stanford.bmir.protege.web.server.filedownload.FileDownloadServlet.doGet(FileDownloadServlet.java:34) at javax.servlet.http.HttpServlet.service(HttpServlet.java:617) at javax.servlet.http.HttpServlet.service(HttpServlet.java:717) at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:487) at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1097) at edu.stanford.bmir.protege.web.server.filter.WebProtegeWebAppFilter.doFilter(WebProtegeWebAppFilter.java:65) at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1088) at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:360) at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216) at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:181) at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:729) at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:405) at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152) at org.mortbay.jetty.handler.RequestLogHandler.handle(RequestLogHandler.java:49) at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152) at org.mortbay.jetty.Server.handle(Server.java:324) at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:505) at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:829) at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:513) at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:211) at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:380) at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:395) at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:488)