Unable To Get Property 'GetControls' Of Undefined Or Null Reference" from pmodel.js

Whenever user gets a warning message to click a button and when user click that button , error occurs in pmodel.js.

In debug window it is observed that "Unable To Get Property 'GetControls' Of Undefined Or Null Reference" is thrown in pmodel.js

Please help to find the cause of this error .

Note : The applet where button is placed has an custom pr file (TestFormAppletPR)

if (typeof(SiebelAppFacade.TestFormAppletPR) === "undefined") {

    SiebelJS.Namespace("SiebelAppFacade.TestFormAppletPR");

    define("siebel/custom/TestFormAppletPR", ["siebel/phyrenderer"], function() {

        SiebelAppFacade.TestFormAppletPR = (function() {

            function TestFormAppletPR(pm) {

                SiebelAppFacade.TestFormAppletPR.superclass.constructor.apply(this, arguments);

            }

            SiebelJS.Extend(TestFormAppletPR, SiebelAppFacade.PhysicalRenderer);

            TestFormAppletPR.prototype.ShowUI = function() {