A <code>W3CEndpointReferenceBuilder</code> instance with
   the extension <code>element</code> added to the
    <code>W3CEndpointReference</code> instance.</dd>
<dt><span class="throwsLabel">Throws:</span></dt>
<dd><code><a href="https://docs.oracle.com/javase/8/docs/api/java/lang/IllegalArgumentException.html?is-external=true" title="class or interface in java.lang">IllegalArgumentException</a></code> - if <code>element</code>
 is <code>null</code>.</dd>
<dt><span class="simpleTagLabel">Since:</span></dt>
