Edittext onChange has undefined value?

I'm trying to add validation to my edittext controls.  I need to make sure they are always numeric values.  I've seen a number of posts indicating various methods for accomplishing this, but my edittext control is always returning an 'undefined' value regardless of whether I add a number or a letter.  I can see the correct value in the debugger's view of the control, but the code fails.  Can anyone please tell me what I'm doing wrong here?

CreateDialog.prototype.run = function()
{
     // declare a bunch of vars
   
    // Create a window of type palette.
    var win = new Window("dialog", "Element Spray Generator",[iTop,iLeft,iTop + iWidth,iLeft + iHeight] );  // bounds = [left, top, right, bottom] 
    this.windowRef = win;

     // add a bunch of other stuff...

    win.txtEditScaleJitter = win.btnPanel.add("edittext",[win.btnPanel.bounds.width/5*4-iPadding, win.txtScaleJitter.bounds.top, win.btnPanel.bounds.width-iPadding, win.txtScaleJitter.bounds.bottom], "100%")

    //Textbox events
    win.txtEditScaleJitter.onChanging = function() {

     //****  here win.txtEditScaleJitter.text is always undefined ******

        var testCharacter = win.txtEditScaleJitter.text.charAt(win.txtEditScaleJitter.text.length -1);
         switch(testCharacter ){
             case "0":  case "1":   case "2": case "3":case "4": case "5":case "6":case "7":case "8": case "9": break;
             default: alert("You entered a character that is not a number  --> " + testCharacter + " <--"); break;
        }
               
    };

   
    // Display the window
    win.show();
       
    return true;   
}