Unable to get property 'hide' of undefined or null reference

Question

My javascript generates an error when I try to close a "ModalPopup" that is not open. (Windows 7 / Visual Studio 2012 / asp.net c# css3 html5) Here is my Javascript ModalPopup Page validator:

<script  type="text/javascript">
var $find= function (id) {
    return document.getElementById(id);
};
   setInterval(closePopup,1000);


    function closePopup() {
      if (!Page_IsValid) {
         $find('mpeScheduleTask').hide();
      }
    }
</script>

Works fine if it's open, generates an error when not open...

Error: 0x800a138f - JavaScript runtime error: Unable to get property 'hide' of undefined or null reference

Answer 1

You can check for its existence first:

function closePopup() {
   if (!Page_IsValid) {
      var mPopup = $find('mpeScheduleTask');
      if (mPopup) mPopup.hide();
    }
}