未捕获的TypeError：无法读取null的属性'offsetHeight'

Question

我正在调用下面文章中描述的SP对话框，但是当我单击“显示对话框”按钮时，我收到以下JS错误。

http://www.a2zmenu.com/Blogs/SharePoint/SharePoint%202010%20Model%20Dialog.aspx

        function OpenDialogPage() {
        var _strdlgtitle = "Add Item";
        var strPageURL = '<%= ResolveClientUrl("~/mm/ItemPopUp.aspx")%>';
        OpenCustomDialog(strPageURL, 500, 390, _strdlgtitle);

        return false;
    }

    function OpenCustomDialog(dialogUrl, dialogWidth, dialogHeight, dialogTitle, dialogAllowMaximize, dialogShowClose) {

        var options = {
            url: dialogUrl,
            allowMaximize: dialogAllowMaximize,
            showClose: dialogShowClose,
            width: dialogWidth,
            height: dialogHeight,
            title: dialogTitle,
            dialogReturnValueCallback: Function.createDelegate(null, CloseCallback
        };
        SP.UI.ModalDialog.showModalDialog(options);
    }

    function CloseCallback(result, returnValue) {
        if (result === SP.UI.DialogResult.OK) {
            __doPostBack('<%= uplPanel.ClientID %>', '');
        }
    }

未捕获的TypeError：无法读取null的属性'offsetHeight'

BTW，SP Dialog正在加载此错误

有什么想法吗？