Calendar generates exception

After clicking on Calendar, huge stack trace is generated in log (full trace attached):

16:57:38,298 ERROR P1-19 http://localhost:8080/xwiki/bin/view/Main/EventCalendar SimpleLog4JLogSystem:logVelocityMessage:154 - Left side ($!hidecolumns) of '==' operation has null value. If a reference, it may not be in the context. Operation not possible. [line 9, column 21]
16:57:38,314 WARN P1-19 http://localhost:8080/xwiki/bin/view/Main/EventCalendar XWikiDocument:display:1276 - Exception showing field title
java.lang.NullPointerException
at com.xpn.xwiki.objects.classes.PropertyClass.displayView(PropertyClass.java:150)
at com.xpn.xwiki.doc.XWikiDocument.display(XWikiDocument.java:1251)
at com.xpn.xwiki.doc.XWikiDocument.display(XWikiDocument.java:1231)
at com.xpn.xwiki.doc.XWikiDocument.display(XWikiDocument.java:1295)
at com.xpn.xwiki.api.Object.get(Object.java:55)
at sun.reflect.GeneratedMethodAccessor59.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.apache.velocity.runtime.parser.node.GetExecutor.execute(GetExecutor.java:111)
at org.apache.velocity.util.introspection.UberspectImpl$VelGetterImpl.invoke(UberspectImpl.java:350)
at org.apache.velocity.runtime.parser.node.ASTIdentifier.execute(ASTIdentifier.java:180)
at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:203)
at org.apache.velocity.runtime.parser.node.ASTReference.render(ASTReference.java:294)
at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:74)
at org.apache.velocity.runtime.directive.Foreach.render(Foreach.java:448)
at org.apache.velocity.runtime.parser.node.ASTDirective.render(ASTDirective.java:170)
at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:318)
at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:231)
at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:134)
at com.xpn.xwiki.render.XWikiVelocityRenderer.render(XWikiVelocityRenderer.java:82)
at com.xpn.xwiki.render.XWikiRenderingEngine.renderText(XWikiRenderingEngine.java:208)
at com.xpn.xwiki.XWiki.include(XWiki.java:2991)
at com.xpn.xwiki.api.XWiki.includeForm(XWiki.java:1298)
at com.xpn.xwiki.api.XWiki.includeForm(XWiki.java:1264)
at sun.reflect.GeneratedMethodAccessor104.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.invoke(UberspectImpl.java:295)
at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:248)
at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:203)
at org.apache.velocity.runtime.parser.node.ASTReference.render(ASTReference.java:294)
at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:318)
at org.apache.velocity.runtime.directive.VelocimacroProxy.render(VelocimacroProxy.java:194)
at org.apache.velocity.runtime.parser.node.ASTDirective.render(ASTDirective.java:170)
at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:318)
at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:231)
at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:134)
at com.xpn.xwiki.render.XWikiVelocityRenderer.render(XWikiVelocityRenderer.java:82)
at com.xpn.xwiki.render.XWikiRenderingEngine.renderText(XWikiRenderingEngine.java:208)
at com.xpn.xwiki.render.XWikiRenderingEngine.renderText(XWikiRenderingEngine.java:131)
at com.xpn.xwiki.render.XWikiRenderingEngine.renderDocument(XWikiRenderingEngine.java:123)
at com.xpn.xwiki.doc.XWikiDocument.getRenderedContent(XWikiDocument.java:350)
at com.xpn.xwiki.api.Document.getRenderedContent(Document.java:334)
at sun.reflect.GeneratedMethodAccessor101.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.invoke(UberspectImpl.java:295)
at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:248)
at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:203)
at org.apache.velocity.runtime.parser.node.ASTReference.render(ASTReference.java:294)
at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:318)
at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:231)
at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:134)
at com.xpn.xwiki.XWiki.parseTemplate(XWiki.java:1218)
at com.xpn.xwiki.XWiki.parseTemplate(XWiki.java:1187)
at com.xpn.xwiki.api.XWiki.parseTemplate(XWiki.java:488)
at sun.reflect.GeneratedMethodAccessor20.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.invoke(UberspectImpl.java:295)
at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:248)
at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:203)
at org.apache.velocity.runtime.parser.node.ASTReference.render(ASTReference.java:294)
at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:318)
at org.apache.velocity.runtime.directive.VelocimacroProxy.render(VelocimacroProxy.java:194)
at org.apache.velocity.runtime.parser.node.ASTDirective.render(ASTDirective.java:170)
at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:74)
at org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfStatement.java:88)
at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:74)
at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:318)
at org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfStatement.java:107)
at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:318)
at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:231)
at com.xpn.xwiki.render.XWikiVelocityRenderer.evaluate(XWikiVelocityRenderer.java:134)
at com.xpn.xwiki.XWiki.parseTemplate(XWiki.java:1218)
at com.xpn.xwiki.XWiki.parseTemplate(XWiki.java:1187)
at com.xpn.xwiki.web.Utils.parseTemplate(Utils.java:98)
at com.xpn.xwiki.web.XWikiAction.execute(XWikiAction.java:159)
at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:431)

at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:236)
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:596)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:689)
at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:428)
at org.mortbay.jetty.servlet.WebApplicationHandler$CachedChain.doFilter(WebApplicationHandler.java:830)
at com.xpn.xwiki.web.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:121)
at org.mortbay.jetty.servlet.WebApplicationHandler$CachedChain.doFilter(WebApplicationHandler.java:821)
at org.mortbay.jetty.servlet.WebApplicationHandler.dispatch(WebApplicationHandler.java:471)
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:568)
at org.mortbay.http.HttpContext.handle(HttpContext.java:1565)
at org.mortbay.jetty.servlet.WebApplicationContext.handle(WebApplicationContext.java:633)
at org.mortbay.http.HttpContext.handle(HttpContext.java:1517)
at org.mortbay.http.HttpServer.service(HttpServer.java:954)
at org.mortbay.http.HttpConnection.service(HttpConnection.java:816)
at org.mortbay.http.HttpConnection.handleNext(HttpConnection.java:983)
at org.mortbay.http.HttpConnection.handle(HttpConnection.java:833)
at org.mortbay.http.SocketListener.handleConnection(SocketListener.java:244)
at org.mortbay.util.ThreadedServer.handle(ThreadedServer.java:357)
at org.mortbay.util.ThreadPool$PoolThread.run(ThreadPool.java:534)