İ want to use this code but when the page opens, I get a null reference error. When I open the page a second time, this error doesn't occur.

I understand when the site caches, this error doesn't occur. The error comes from tb_show function.


My code:
<script type="text/javascript">

function writeCookie(CookieAdi) {
var dtGun = 1
var strValue = "1"
if (dtGun == null || dtGun == "") dtGun = 365;
var d = new Date();
d.setTime(d.getTime() + (dtGun * 24 * 60 * 60 * 1000));
var zt = "; expires=" + d.toGMTString();
document.cookie = CookieAdi + "=" + strValue + zt + "; path=/";
}

function readCookie(cookieadi) {
var c = document.cookie;
if (c.indexOf(cookieadi) != -1) {
s1 = c.indexOf("=", c.indexOf(cookieadi)) + 1;
s2 = c.indexOf(";", s1);
if (s2 == -1) s2 = c.length;
strValue = c.substring(s1, s2);
return strValue;
}
}


writeCookie('OnerFacebook');


if (readCookie('OnerFacebook') != 1) {
tb_show('', 'http://www.mobilyala.com/OnerFacebook/?KeepThis=true&TB_iframe=true&height=500&width=300&modal=true', '');
}


</script>

What should I do for it?