title: Zk 5.0.11 NullPointerException when using fulfill attribute inside listbox
body: <p>when using a simple example of fulfill, inside a listbox, I get a null pointer exception, which I can't get rid of.
  <a href="http://zkfiddle.org/sample/38ipduo/44-Listbox-fulfill-attr-error" rel="nofollow">http://zkfiddle.org/sample/38ipduo/44-Listbox-fulfill-attr-error</a></p>
  
  <p>The fiddle above provides a working example of the error. 
  Debugging the code, I can see the next line</p>
  
  <pre><code>org.zkoss.zk.ui.impl.UiEngineImpl$FulfillListener.&lt;init&gt;(UiEngineImpl.java:2058)
  </code></pre>
  
  <p>being called twice. The first time everything seems fine, but the second time it seems some data is not filled in, causing the NPE.
  Any ideas how can I work the fiddle work correctly? 
  Man thanks in advance.</p>
  
  <p>e.g.</p>
  
  <pre><code>&lt;listcell&gt;
    &lt;button id="btn" label="show" &gt;&lt;/button&gt;
        &lt;div fulfill="btn.onClick"&gt;
        Hello world
        &lt;/div&gt;
    &lt;textbox value="@{person.name}" inplace="true" &gt;&lt;/textbox&gt;
  &lt;/listcell&gt;
  </code></pre>
  
  <p>I'm trying a list with a single record, as one initial answer in the zk forums 
  suggested several id="btn" elements may be causing conflicts. THe fiddle is using zk 5. I have tried with 6.5 and got a similar error.</p>