title: what is wrong in my java zkoss program
body: <pre><code>updated
  
  &lt;?page title="example"?&gt;
  &lt;window  id="music" apply="com.main"&gt;
  
  &lt;combobox id="combo" autodrop="true" onChanging="music.suggest()"/&gt;
  
  &lt;/window&gt;
  java
  public class main extends GenericComposer{
  
   /**
    * 
    */
  
   private static final long serialVersionUID = 1L;
   Combobox combo;
   public void suggest() {
    combo.getFellow("combo");
  
    combo.getItems().clear();
  
              combo.appendItem("Ace");
              combo.appendItem("Ajax");
              combo.appendItem("Apple");
  
              combo.appendItem("Best");
              combo.appendItem("Blog");
  
   }
  
   }
  </code></pre>
  
  <p>it says null pointer exception y???</p>