Cannot Call method 'Ajax Request' is null

function PriOnClientDropDownClosing(sender, eventArgs) {
try {
var Pri = document.getElementById("Pri");
Pri.onclick = StopPropagation;
var PricomboBoxClicked = $find("<%= cbxPrimary.ClientID %>");
if (PricomboBoxClicked.get_text() != "" && PricomboBoxClicked.get_value() != "") {
$find("<%= ajxpnlSec.ClientID %>").ajaxRequest();
}
}
catch (err) {
alert(err);
}
}