java - 异常 : source is null for getProperty(null, "agent")

Question

我刚刚学习如何使用 Tapestry，但不断遇到以下错误

Exception: source is null for getProperty(null, "agent")    
ognl.OgnlRuntime.getProperty(OgnlRuntime.java:2203)

这是我的 html 页面中的代码

<tr>
          <td valign="center" align="right" height="30"><span class="label1"> <span key="agentDeletionSalesParentActivityType" />&nbsp;:&nbsp;</span></td>
          <td valign="center" align="left" height="30"><select jwcid="agentDeletionSalesParentActivityType@ActivityTypePropertySelection" regions="ognl:{activity.agent.region}" emptyOption="true" disabled="ognl:disabled" value="ognl:parentActivityType" /></td>
        </tr>
          <tr>
            <td valign="center" align="right" height="30"><span class="label1"> <span key="agentDeletionSalesChildActivityType" />&nbsp;:&nbsp;</span></td>
            <td valign="center" align="left" height="30"><select jwcid="agentDeletionSalesChildActivityType" regions="ognl:{activity.agent.region}" emptyOption="true" disabled="ognl:disabled" value="ognl:childActivityType" parent="component:agentDeletionSalesParentActivityType" /></td>
          </tr>

这是我的java代码

@Persist
public abstract ActivityType getParentActivityType();
public abstract void setParentActivityType(ActivityType parent);

@Persist
public abstract ActivityType getChildActivityType();
public abstract void setChildActivityType(ActivityType child);

@Component(id="agentDeletionSalesChildActivityType", type="ActivityTypePropertySelection")
public abstract ActivityTypePropertySelection getChildActivityPropertySelection();

如果您需要更多代码，我很乐意将其放在上面(保持较小，以免将代码堆积到页面中)，任何人都可以看出为什么会返回此异常吗？

Answer 1

这似乎是说 activity 对象在此为空:

regions="ognl:{activity.agent.region}"