Adding trigger causes "Control is null" Javascript error

Hello,
   I have an XForm with a trigger which shows a modal dialog. On the dialog is another trigger. This trigger causes a javascript error 'control is null' when the form is rendered. The trigger is defined as follows:

<xhtml:div id='DocletRecordModal' >
    <xhtml:div class="modalbuttons clearfix">
        <xf:trigger xhtml:class="edit">
            <xf:label>Edit</xf:label>
            <xforms:action ev:event="DOMActivate">
                <xxforms:script>
                    alert("Editing");
                </xxforms:script>
            </xforms:action>
        </xf:trigger>
    </xhtml:div>
</xhtml:div>

As soon as the modal is shown, the error occurs. I am using firebug to examine any javascript errors.

Please help me to resolve this before I pull what little hair I have left out.
Regards
Peter Phillips