(only when listbox + CE) EE doesn't have this issue
the code is simulating mvvm behavior

<zk>
	<label multiline="true">
(only when listbox + CE) EE doesn't have this issue
the code is simulating mvvm behavior
	1.chick addToSelection you should see a item A add to next page without an exception
	2.this exception cause by a wrong implementation of object's hashcode()
	3.should consider to avoid this exception (eat at org.zkoss.zul.Listbox.setSelectedIndex(Listbox.java:929)(need to check side effect)
	  or throw meaningful exception(better)
	</label>
	<div width="400px" >
		<listbox id="listbox" mold="paging" pageSize="10">
			<listhead>
				<listheader label="name" />
			</listhead>
			<template name="model" >
				<listitem>
					<listcell label="${each.name}" />
				</listitem>
			</template>
		</listbox>
		<button label="addToSelection" onClick="addToSelection()"/>
	</div>
<zscript><![CDATA[
	class Item {
		String name;
		public Item() {
			
		}
		public Item(String name) {
			this.name = name;
		}

		public String getName() {
			return name;
		}

		public void setName(String name) {
			this.name = name;
		}
		
		public int hashCode() {
			final int prime = 31;
			int result = 1;
			result = prime * result + ((name == null) ? 0 : name.hashCode());
			return result;
		}

		
		public boolean equals(Object obj) {
			if (this == obj)
				return true;
			if (obj == null)
				return false;
			if (!(obj instanceof Item))
				return false;
			Item other = (Item) obj;
			if (name == null) {
				if (other.name != null)
					return false;
			} else if (!name.equals(other.name))
				return false;
			return true;
		}
		
	}
	List itemList = new LinkedList();
	for (int i = 0; i < 10; i++) {
		itemList.add(new Item("item " + i));
	}
	ListModelList listModelList = new ListModelList(itemList,true);//if use live list
	listbox.setModel(listModelList);
	
	//1.an item is not in collection yet, however, it already in the selection
	Item n = new Item();
	listModelList.addSelection(n);//un non in list selection

	public void addToSelection() {
		//2.after the value changed of the item
		//the wrong imple of item hash cause this exception
		n.setName("A");
		itemList.add(n);
		listModelList.addSelection(n);
		listbox.setModel(listModelList);
	}
]]>
</zscript>
</zk>

>>java.lang.NullPointerException
>> at org.zkoss.zul.Listbox.setSelectedIndex(Listbox.java:929)
>> at org.zkoss.zul.Listbox.onListDataChange(Listbox.java:2531)
>> at org.zkoss.zul.Listbox.access$17(Listbox.java:2521)
>> at org.zkoss.zul.Listbox$4.onChange(Listbox.java:461)
>> at org.zkoss.zul.AbstractListModel.fireEvent(AbstractListModel.java:61)
>> at org.zkoss.zul.ListModelList.fireSelectionEvent(ListModelList.java:438)
>> at org.zkoss.zul.AbstractListModel.addToSelection(AbstractListModel.java:130)
>> at org.zkoss.zul.ListModelList.addSelection(ListModelList.java:445)
>> at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
>> at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
>> at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
>> at java.lang.reflect.Method.invoke(Method.java:597)
>> at bsh.Reflect.invokeMethod(Unknown Source)
>> at bsh.Reflect.invokeObjectMethod(Unknown Source)
>> at bsh.Name.invokeMethod(Unknown Source)
>> at bsh.BSHMethodInvocation.eval(Unknown Source)
>> at bsh.BSHPrimaryExpression.eval(Unknown Source)
>> at bsh.BSHPrimaryExpression.eval(Unknown Source)
>> at bsh.BSHBlock.evalBlock(Unknown Source)
>> at bsh.BSHBlock.eval(Unknown Source)
>> at bsh.BshMethod.invokeImpl(Unknown Source)
>> at bsh.BshMethod.invoke(Unknown Source)
>> at bsh.BshMethod.invoke(Unknown Source)
>> at bsh.Name.invokeLocalMethod(Unknown Source)
>> at bsh.Name.invokeMethod(Unknown Source)
>> at bsh.BSHMethodInvocation.eval(Unknown Source)
>> at bsh.BSHPrimaryExpression.eval(Unknown Source)
>> at bsh.BSHPrimaryExpression.eval(Unknown Source)
>> at bsh.Interpreter.eval(Unknown Source)
>> at bsh.Interpreter.eval(Unknown Source)
>> at org.zkoss.zk.scripting.bsh.BSHInterpreter.exec(BSHInterpreter.java:131)
>> at org.zkoss.zk.scripting.util.GenericInterpreter.interpret(GenericInterpreter.java:328)
>> at org.zkoss.zk.ui.impl.PageImpl.interpret(PageImpl.java:875)
>> at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2701)
>> at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2654)
>> at org.zkoss.zk.ui.impl.EventProcessor.process(EventProcessor.java:136)
>> at org.zkoss.zk.ui.impl.UiEngineImpl.processEvent(UiEngineImpl.java:1710)
>> at org.zkoss.zk.ui.impl.UiEngineImpl.process(UiEngineImpl.java:1495)
>> at org.zkoss.zk.ui.impl.UiEngineImpl.execUpdate(UiEngineImpl.java:1205)
>> at org.zkoss.zk.au.http.DHtmlUpdateServlet.process(DHtmlUpdateServlet.java:583)
>> at org.zkoss.zk.au.http.DHtmlUpdateServlet.doGet(DHtmlUpdateServlet.java:481)
>> at org.zkoss.zk.au.http.DHtmlUpdateServlet.doPost(DHtmlUpdateServlet.java:489)
>> at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
>> at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
>> at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
>> at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:401)
>> at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
>> at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
>> at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:766)
>> at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:450)
>> at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
>> at org.mortbay.jetty.Server.handle(Server.java:326)
>> at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
>> at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:945)
>> at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:756)
>> at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:218)
>> at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
>> at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:410)
>> at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582)