this._postBackSettings.async is null or not an object

You are using a PopupControlExtender from the AjaxControlToolkit and you added a Button or an ImageButton in it and every time you click on it this ugly js error appears: this._postBackSettings.async is null or not an object. You Google and you see that the solution is to use a LinkButton instead of a Button or an ImageButton.

Well, I want my ImageButton! Therefore I added to the Page_Load method of my page or control this little piece of code to fix the bug:

private void FixPopupFormSubmit()
{
  var script =
    @"if( window.Sys && Sys.WebForms 
    && Sys.WebForms.PageRequestManager 
	&& Sys.WebForms.PageRequestManager.getInstance) {
  var prm = Sys.WebForms.PageRequestManager.getInstance();
  if (prm && !prm._postBackSettings) {
    prm._postBackSettings = prm._createPostBackSettings(false, null, null);
  }
}";
  ScriptManager.RegisterOnSubmitStatement(
    Page, 
    Page.GetType(), 
    "FixPopupFormSubmit", 
    script);
}

Hope it helps you all.