FreeMarker template error

The following has evaluated to null or missing:
==> _category  [in template "sitemap.html" at line 6, column 10]

----
Tip: If the failing expression is known to be legally refer to something that's null or missing, either specify a default value like myOptionalVar!myDefault, or use <#if myOptionalVar??>when-present<#else>when-missing</#if>. (These only cover the last step of the expression; to cover the whole expression, use parenthesis: (myOptionalVar.foo)!myDefault, (myOptionalVar.foo)??
----

----
FTL stack trace ("~" means nesting-related):
	- Failed at: ${_category.title?html}  [in template "sitemap.html" at line 6, column 8]
----

Java stack trace (for programmers):
----
freemarker.core.InvalidReferenceException: [... Exception message was already printed; see it above ...]
	at freemarker.core.InvalidReferenceException.getInstance(InvalidReferenceException.java:108)
	at freemarker.core.UnexpectedTypeException.newDesciptionBuilder(UnexpectedTypeException.java:60)
	at freemarker.core.UnexpectedTypeException.<init>(UnexpectedTypeException.java:40)
	at freemarker.core.NonHashException.<init>(NonHashException.java:46)
	at freemarker.core.Dot._eval(Dot.java:45)
	at freemarker.core.Expression.eval(Expression.java:76)
	at freemarker.core.Expression.evalAndCoerceToString(Expression.java:80)
	at freemarker.core.BuiltInForString._eval(BuiltInForString.java:26)
	at freemarker.core.Expression.eval(Expression.java:76)
	at freemarker.core.Expression.evalAndCoerceToString(Expression.java:80)
	at freemarker.core.DollarVariable.accept(DollarVariable.java:40)
	at freemarker.core.Environment.visit(Environment.java:257)
	at freemarker.core.MixedContent.accept(MixedContent.java:57)
	at freemarker.core.Environment.visit(Environment.java:257)
	at freemarker.core.Environment.process(Environment.java:235)
	at freemarker.template.Template.process(Template.java:262)
	at freemarker.ext.servlet.FreemarkerServlet.process(FreemarkerServlet.java:420)
	at freemarker.ext.servlet.FreemarkerServlet.doGet(FreemarkerServlet.java:359)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:291)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
	at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:197)
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:219)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:142)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:518)
	at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1091)
	at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:668)
	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1527)
	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1484)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
	at java.lang.Thread.run(Thread.java:745)

亚洲综合婷婷六月丁_一本道av影音先锋影院