JavaScript document.getElementById returns NULL when using ClientScript.RegisterStartupScript in ASP.Net

Hi aal of you,

When I am writing JavaScript in C# and using it on body onload. It is not working.

My C# Code is :

string csName = "display";
            Type csType = this.GetType();
            ClientScriptManager csm = Page.ClientScript;
            if (!csm.IsStartupScriptRegistered(csType, csName))
            {
                sb.Append("<script type='text/javascript'>");
                sb.Append("var Pics = new Array();");
                sb.Append("var Index = 0;");
                sb.Append("var l = 0;");

                sb.Append("var sec = 60;");
                sb.Append("var min = " + Session["mm"] + ";");
                sb.Append("var hour = 0;");
                sb.Append("var t;");
                sb.Append("function display(){");
                sb.Append("sec -= 1;");
                sb.Append("if ((sec == 0) && (min == 0) && (hour == 0)){");
                sb.Append("document.getElementById('<%=hd.ClientID%>').value='1';");
                sb.Append("document.getElementById('<%=bsubmit.ClientID%>').click();");
                sb.Append("return;}");
                sb.Append("if (sec < 0){");
                sb.Append("sec = 59;");
                sb.Append("min -= 1;}");
                sb.Append("if (min < 0){");
                sb.Append("min = 59;");
                sb.Append("hour -= 1;}");
                sb.Append("else {");
                sb.Append("document.getElementById('<%=tTimer.ClientID%>').value = hour + ':' + min + ':' + sec;");
                sb.Append("t = setTimeout('display()', 1000);}");
                
                int i, j;
                for (i = 0; i <= DtFill.Rows.Count - 1; i++)
                {
                    j = i + 1;
                    sb.Append("Pics[" + i + "] = new Image();");
                    sb.Append("Pics[" + i + "].src='" + PID + "/" + DtFill.Rows[i]["QsnImage"] + "';");
                }
                sb.Append("if(l==0) {");
                sb.Append("document.getElementById('IQsnImage').src = Pics[0].src;");
                sb.Append("}");
                sb.Append("}");
                sb.Append("window.onload = display;");
                sb.Append("alert('Lokesh Hammad');");
                sb.Append("</script>");
                S1 = sb.ToString();

                //if (!Page.ClientScript.IsClientScriptBlockRegistered(Page.GetType(), "JSScript")) ClientScript.RegisterClientScriptBlock(Page.GetType(), "JSScript", S1.ToString());
                //csm.RegisterStartupScript(csType, csName, S1.ToString());
                Page page = HttpContext.Current.CurrentHandler as Page;
                page.ClientScript.RegisterStartupScript(typeof(Page), "Test", sb.ToString());
            }

Please help me for it.