ZK取得某列某listcell的id 錯誤

<?page title="Data binding 5" contentType="text/html;charset=UTF-8"?>
<?init class="org.zkoss.zkplus.databind.AnnotateDataBinderInit" ?>
<window id="win" title="Data binding 5" border="normal" use="com.sanq.test.Listbox_binding_win">
  <zscript><![CDATA[
     //prepare the sample List
        import com.sanq.test.Person;
        int count = 3;
        List persons = new ArrayList();
        for(int j= 1; j <= count; ++j) {
            Person personx = new Person();
            personx.setFirstName("Tony"+j);
            personx.setLastName(""+j);     
            persons.add(personx);
        }
     ]]></zscript>
  
    <listbox rows="4" model="@{persons}">
        <listhead>
            <listheader label="First Name" width="100px" />
            <listheader label="Last Name" width="100px" />
            <listheader label="Full Name" width="100px" />
        </listhead>
        <!-- define variable person here-->
        <listitem self="@{each='person'}">
            <listcell>
                <textbox id="id_first" value="@{person.firstName}" readonly="true"/>
            </listcell>
            <listcell>
                <textbox id="id_last" value="@{person.lastName}" readonly="true"/>
            </listcell>
            <listcell id="id_full" label="@{person.fullName}"/>
        </listitem>
    </listbox>
    <separator bar="false"/>
          <hbox>
           <button label="修改F3" onClick="win.onModify()"/>
         </hbox>
</window>

package com.sanq.test;
 
import org.zkoss.zul.Textbox;
import org.zkoss.zul.Window;
 
public class Listbox_binding_win extends Window {
 
  public void onModify() {
    Textbox tb = (Textbox) getFellow("id_first");
    //tb.setReadonly(false);
    //tb.setFocus(true);
  }
}