im getting error 'parent.frames.schTop.document' is null or not an object

My code is
if (gFlag == "PATINFO") {
var CalendarString = "";
if (returnString != "") {
var str11 = parent.frames["schBanner"].document.location.href;
parent.frames["schBanner"].document.getElementById("hdnFromScduler").value = top.frames["banner"].document.submenu.hidModPatId.value;
//alert(parent.frames["schBanner"].document.getElementById("hdnFromScduler").value);

if (parent.document.getElementById("tblFrame") != null) {
parent.document.getElementById("tblFrame").outerHTML = "";
}
if (parent.document.getElementById("PatTblInfo") != null) {
parent.document.getElementById("PatTblInfo").outerHTML = "";
}
if (parent.document.getElementById("divDisplay") != null) {
parent.document.getElementById("divDisplay").innerHTML = "";
}
parent.document.getElementById("spnPatientInfo").style.display = "";
parent.document.getElementById("spnPatientInfo").innerHTML = "";
parent.document.getElementById("spnPatientInfo").innerHTML = returnString;

}
}

How to solve this.