https://sourceforge.net/tracker/?func=detail&aid=2931804&group_id=152762&atid=785191

hello, I'm trying to transit to a sub-flow state but facing problems.
Below is my configuration:
First, the zul page:

<?init class="org.zkoss.zwf.FlowHandler" arg0="/WEB-INF/flow/functions/function1/index.xml"?>
<?variable-resolver class="org.zkoss.zkplus.spring.DelegatingVariableResolver" ?>
<zul:zk xmlns="http://www.zkoss.org/2005/zk/native"
xmlns:zul="http://www.zkoss.org/2005/zul"
xmlns:a="http://www.zkoss.org/2005/zk/annotation"
xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'
xsi:schemaLocation='http://www.zkoss.org/2005/zul http://www.zkoss.org/2005/zul/zul.xsd'>
<zul:zscript>
</zul:zscript>
<zul:window border="none" width="100%" height="100%">
<zul:div width="100%" height="100%" self="@

"/>
</zul:window>
</zul:zk>

Then, the main flow definition:

<?xml version="1.0" encoding="UTF-8"?>
<flow id="test1">

<subflow-state id="subflow" subflow="/WEB-INF/subflow1">
<transition to="finish" onTransit='finish' />
</subflow-state>

<end-state id="finish" />
</flow>

Finally, the sub flow definition:

<?xml version="1.0" encoding="UTF-8"?>
<flow id="fullContainerFlow">
<end-state id="finish" />
</flow>

As shown above, the flow definition is fairly simple. However, I always get the following exception:

java.lang.NullPointerException
>> at org.zkoss.zwf.impl.SubflowStateImpl.enter(SubflowStateImpl.java:49)
>> at org.zkoss.zwf.impl.FlowImpl.gotoState(FlowImpl.java:407)
>> at org.zkoss.zwf.impl.FlowImpl.enter(FlowImpl.java:339)
>> at org.zkoss.zwf.impl.FlowImpl.restore(FlowImpl.java:626)
>> at org.zkoss.zwf.FlowHandler$FlowBookMarkChangeListener.onEvent(FlowHandler.java:195)
>> at org.zkoss.zk.ui.impl.EventProcessor.process0(EventProcessor.java:168)
>>...