ZK org.zkoss.zk.ui.UiException: Only one child and one caption is allowed: <Center q8BP5>

ZK <borderlayout>中的<north>，<west>，<east>，<south>，<center>都只能有一個子原件（除了<caption>），否則會出現下面的錯誤：

org.zkoss.zk.ui.UiException: Only one child and one caption is allowed

例如下面的<north>中有兩個<label>，就會引發此錯誤。

<borderlayout>
    <north>
        <label>LABLE 1</label>
        <label>LABLE 2</label>
    </north>
    ...
</borderlayout>