java.lang.NullPointerException: This exception has no message.

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java.lang.NullPointerException:
at edu.isi.karma.kr2rml.KR2RMLMappingWriter.addCompleteWorksheetHistory(KR2RMLMappingWriter.java:435)
at edu.isi.karma.kr2rml.KR2RMLMappingWriter.addKR2RMLMappingResource(KR2RMLMappingWriter.java:115)
at edu.isi.karma.kr2rml.KR2RMLMappingWriter.addR2RMLMapping(KR2RMLMappingWriter.java:141)
at edu.isi.karma.controller.command.alignment.R2RMLAlignmentFileSaver.writeModel(R2RMLAlignmentFileSaver.java:127)
at edu.isi.karma.controller.command.alignment.R2RMLAlignmentFileSaver.saveAlignment(R2RMLAlignmentFileSaver.java:107)
at edu.isi.karma.controller.command.alignment.R2RMLAlignmentFileSaver.saveAlignment(R2RMLAlignmentFileSaver.java:79)
at edu.isi.karma.controller.command.alignment.GenerateR2RMLModelCommand.doIt(GenerateR2RMLModelCommand.java:284)
at edu.isi.karma.controller.history.CommandHistory.doCommand(CommandHistory.java:162)
at edu.isi.karma.controller.history.CommandHistory.doCommand(CommandHistory.java:130)
at edu.isi.karma.webserver.ExecutionController.invokeCommand(ExecutionController.java:129)
at edu.isi.karma.webserver.RequestController.doPost(RequestController.java:99)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:755)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:848)
at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:698)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:505)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:138)
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:564)
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:213)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1096)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:432)
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:175)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1030)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:136)
at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:201)
at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:109)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:97)
at org.eclipse.jetty.server.Server.handle(Server.java:445)
at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:268)
at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:229)
at org.eclipse.jetty.io.AbstractConnection$ReadCallback.run(AbstractConnection.java:358)
at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:601)
at java.lang.Thread.run(Thread.java:745)

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