Modal Pop Extender Inside Update Panel?

I have a modal popup extender which is triggered by gridview onRowCommand. The gridview and the modal popup extender is in different updatePanel. The panel which assigned to PopupControlID of the mpe contains a textbox and a button. This button will triggered a page method web service that will return a value to be assigned to the textbox. My problem is when I click the button, after partial post back, the mpe disapper (hide). I tried everything to make it show. Below is my code.

function Completed(result) {
//get the target textbox inside the mpe
var txt = '<%= txtContractNo.ClientID %>';
var txtRef = document.getElementById(txt);
txtRef.value = result;
var mpeId = '<%= mpeContract.ClientID %>';
$find(mpeId).show();
}

How to make it show after partial post back?