Steps to Reproduce

the zul

<zk>
    <include src="${'empty.zul'}"/>
</zk>

This is a way to simulate multi-threading

set 2 breakpoints

breakpoint A
org.zkoss.zk.xel.ExValue: 93

breakpoint B
ExValue: 110

Suspend by thread

Steps

1. visit zul with chrome and stop at breakpoint A. This is thread A.
2. visit zul with another browser (firefox) and stop at breakpoint B, step to line 111. This is thread B.
3. let thread A step one line to set _expr = null
4. let thread B run eval.evaluate()

Current Result

9.6.1
java.lang.NullPointerException
org.zkoss.zk.xel.impl.SimpleEvaluator.evaluate(SimpleEvaluator.java:70)
org.zkoss.zk.ui.metainfo.AbstractEvalRef.evaluate(AbstractEvalRef.java:46)
org.zkoss.zk.xel.ExValue.getValue(ExValue.java:111)
org.zkoss.zk.ui.metainfo.Property.getValue(Property.java:138)
org.zkoss.zk.ui.metainfo.Property.assign0(Property.java:272)
org.zkoss.zk.ui.metainfo.Property.assign(Property.java:174)
org.zkoss.zk.ui.metainfo.ComponentInfo.applyProperties(ComponentInfo.java:742)
org.zkoss.zk.ui.impl.AbstractUiFactory.newComponent(AbstractUiFactory.java:150)
org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild0(UiEngineImpl.java:921)
org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild(UiEngineImpl.java:891)
org.zkoss.zk.ui.impl.UiEngineImpl.execCreate0(UiEngineImpl.java:778)
org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild(UiEngineImpl.java:838)
org.zkoss.zk.ui.impl.UiEngineImpl.execCreate0(UiEngineImpl.java:796)
org.zkoss.zk.ui.impl.UiEngineImpl.execCreate(UiEngineImpl.java:742)
org.zkoss.zk.ui.impl.UiEngineImpl.execNewPage0(UiEngineImpl.java:463)
org.zkoss.zk.ui.impl.UiEngineImpl.execNewPage(UiEngineImpl.java:371)
org.zkoss.zk.ui.http.DHtmlLayoutServlet.process(DHtmlLayoutServlet.java:220)
org.zkoss.zk.ui.http.DHtmlLayoutServlet.doGet(DHtmlLayoutServlet.java:146)

8.6.2.1
java.lang.NullPointerException
at org.zkoss.zk.xel.impl.SimpleEvaluator.evaluate(SimpleEvaluator.java:68)
at org.zkoss.zk.ui.metainfo.AbstractEvalRef.evaluate(AbstractEvalRef.java:46)
at org.zkoss.zk.xel.ExValue.getValue(ExValue.java:111)
at org.zkoss.zk.ui.metainfo.Property.getValue(Property.java:138)
at org.zkoss.zk.ui.metainfo.Property.assign0(Property.java:272)
at org.zkoss.zk.ui.metainfo.Property.assign(Property.java:174)
at org.zkoss.zk.ui.metainfo.ComponentInfo.applyProperties(ComponentInfo.java:742)
at org.zkoss.zk.ui.impl.AbstractUiFactory.newComponent(AbstractUiFactory.java:146)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild0(UiEngineImpl.java:918)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild(UiEngineImpl.java:888)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreate0(UiEngineImpl.java:775)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreate(UiEngineImpl.java:739)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild0(UiEngineImpl.java:933)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild(UiEngineImpl.java:888)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreate0(UiEngineImpl.java:775)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreateChild(UiEngineImpl.java:835)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreate0(UiEngineImpl.java:793)
at org.zkoss.zk.ui.impl.UiEngineImpl.execCreate(UiEngineImpl.java:739)
at org.zkoss.zk.ui.impl.UiEngineImpl.execNewPage0(UiEngineImpl.java:460)
at org.zkoss.zk.ui.impl.UiEngineImpl.execNewPage(UiEngineImpl.java:368)
at org.zkoss.zk.ui.http.DHtmlLayoutServlet.process(DHtmlLayoutServlet.java:215)
at org.zkoss.zk.ui.http.DHtmlLayoutServlet.doGet(DHtmlLayoutServlet.java:140

Expected Result

no exception

Debug Information

• the real case is web testing with many parallel requests, randomly happen

Workaround