VelocityServlet: Error processing the template

Invocation of method 'generateDisplaysRoomURL' in  class uk.org.tate.application.collections.URLMaker threw exception java.lang.NullPointerException at templates/collection/Breadcrumb.html[line 173, column 48]
org.apache.velocity.exception.MethodInvocationException: Invocation of method 'generateDisplaysRoomURL' in  class uk.org.tate.application.collections.URLMaker threw exception java.lang.NullPointerException at templates/collection/Breadcrumb.html[line 173, column 48]
	at org.apache.velocity.runtime.parser.node.ASTMethod.handleInvocationException(ASTMethod.java:337)
	at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:284)
	at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:262)
	at org.apache.velocity.runtime.parser.node.ASTReference.value(ASTReference.java:507)
	at org.apache.velocity.runtime.parser.node.ASTExpression.value(ASTExpression.java:71)
	at org.apache.velocity.runtime.parser.node.ASTSetDirective.render(ASTSetDirective.java:142)
	at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:72)
	at org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfStatement.java:87)
	at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:72)
	at org.apache.velocity.runtime.parser.node.ASTElseIfStatement.render(ASTElseIfStatement.java:92)
	at org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfStatement.java:106)
	at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:336)
	at org.apache.velocity.runtime.directive.Parse.render(Parse.java:260)
	at org.apache.velocity.runtime.parser.node.ASTDirective.render(ASTDirective.java:175)
	at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:336)
	at org.apache.velocity.runtime.directive.Parse.render(Parse.java:260)
	at org.apache.velocity.runtime.parser.node.ASTDirective.render(ASTDirective.java:175)
	at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:336)
	at org.apache.velocity.Template.merge(Template.java:328)
	at org.apache.velocity.Template.merge(Template.java:235)
	at org.apache.velocity.servlet.VelocityServlet.mergeTemplate(VelocityServlet.java:452)
	at uk.org.tate.core.servlet.TateServlet.processRequest(TateServlet.java:482)
	at uk.org.tate.core.servlet.TateServlet.mergeTemplate(TateServlet.java:427)
	at org.apache.velocity.servlet.VelocityServlet.doRequest(VelocityServlet.java:379)
	at uk.org.tate.core.servlet.TateServlet.doRequest(TateServlet.java:305)
	at org.apache.velocity.servlet.VelocityServlet.doGet(VelocityServlet.java:318)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:689)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
	at org.apache.catalina.servlets.InvokerServlet.serveRequest(InvokerServlet.java:419)
	at org.apache.catalina.servlets.InvokerServlet.doGet(InvokerServlet.java:133)
	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 com.opensymphony.module.sitemesh.filter.PageFilter.parsePage(PageFilter.java:118)
	at com.opensymphony.module.sitemesh.filter.PageFilter.doFilter(PageFilter.java:52)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:204)
	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.valves.AccessLogValve.invoke(AccessLogValve.java:541)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
	at org.apache.jk.server.JkCoyoteHandler.invoke(JkCoyoteHandler.java:199)
	at org.apache.jk.common.HandlerRequest.invoke(HandlerRequest.java:282)
	at org.apache.jk.common.ChannelSocket.invoke(ChannelSocket.java:754)
	at org.apache.jk.common.ChannelSocket.processConnection(ChannelSocket.java:684)
	at org.apache.jk.common.ChannelSocket$SocketConnection.runIt(ChannelSocket.java:876)
	at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
	at java.lang.Thread.run(Thread.java:619)
Caused by: java.lang.NullPointerException
	at uk.org.tate.application.collections.URLMaker.generateDisplaysRoomURL(URLMaker.java:1413)
	at sun.reflect.GeneratedMethodAccessor643.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.doInvoke(UberspectImpl.java:389)
	at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.invoke(UberspectImpl.java:378)
	at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:270)
	... 50 more