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 (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.