FreeMarker template error:
The following has evaluated to null or missing:
==> arts[0]  [in template "wap_page.ftl" at line 3, column 37]

Tip: If the failing expression is known to be legally null/missing, either specify a default value with 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 parenthessis: (myOptionVar.foo)!myDefault, (myOptionVar.foo)??

The failing instruction (FTL stack trace):
----------
==> #assign art = articleTag("{\'aid\':" ...  [in template "wap_page.ftl" at line 3, column 1]
----------

Java stack trace (for programmers):
----------
freemarker.core.InvalidReferenceException: [... Exception message was already printed; see it above ...]
	at freemarker.core.InvalidReferenceException.getInstance(InvalidReferenceException.java:98)
	at freemarker.core.UnexpectedTypeException.newDesciptionBuilder(UnexpectedTypeException.java:41)
	at freemarker.core.UnexpectedTypeException.<init>(UnexpectedTypeException.java:24)
	at freemarker.core.Dot._eval(Dot.java:81)
	at freemarker.core.Expression.eval(Expression.java:111)
	at freemarker.core.AddConcatExpression._eval(AddConcatExpression.java:92)
	at freemarker.core.Expression.eval(Expression.java:111)
	at freemarker.core.AddConcatExpression._eval(AddConcatExpression.java:91)
	at freemarker.core.Expression.eval(Expression.java:111)
	at freemarker.core.Expression.evalAndCoerceToString(Expression.java:115)
	at freemarker.core.ListLiteral.getValueList(ListLiteral.java:103)
	at freemarker.core.MethodCall._eval(MethodCall.java:94)
	at freemarker.core.Expression.eval(Expression.java:111)
	at freemarker.core.Assignment.accept(Assignment.java:106)
	at freemarker.core.Environment.visit(Environment.java:265)
	at freemarker.core.MixedContent.accept(MixedContent.java:93)
	at freemarker.core.Environment.visit(Environment.java:265)
	at freemarker.core.Environment.process(Environment.java:243)
	at freemarker.template.Template.process(Template.java:277)
	at com.paidot.ocms.utils.FreemarkerUtils.exportHtml(FreemarkerUtils.java:99)
	at com.paidot.ocms.utils.FreemarkerUtils.exportIndexHtml(FreemarkerUtils.java:278)
	at com.paidot.ocms.utils.FreemarkerUtils.exportCategory(FreemarkerUtils.java:239)
	at com.paidot.ocms.controller.admin.StaticController$1.run(StaticController.java:203)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)