How to prevent Javascript alert box message "prevent this page from creating additional dialog"?

<script type="text/javascript">

            function isNumberKey(evt) {

                var charCode = (evt.which) ? evt.which : event.keyCode
                if ((charCode >= 48 && charCode <= 57) || (charCode == 8)) {
                    return true;
                }
                else {
                    return false;
                }
            }

            function isNumberKey2(evt) {
                var charCode = (evt.which) ? evt.which : event.keyCode
                if ((charCode >= 48 && charCode <= 57) || (charCode == 8) || (charCode == 32)) {
                    return true;
                }
                else {
                    return false;
                }
            }
            function isNumberKey1(evt) {
                var charCode = (evt.which) ? evt.which : event.keyCode
                if ((charCode >= 48 && charCode <= 57) || (charCode == 8) || (charCode == 46)) {
                    return true;
                }
                else {
                    return false;
                }
            }
            function validatentry() {

                var txtdate = document.getElementById('<%=txtdate.ClientID%>').value;
                var txtagency_name = document.getElementById('<%=txtagency_name.ClientID%>').value;
                var txtagency_code = document.getElementById('<%=txtagency_code.ClientID%>').value;
                var txtagency_ACcode = document.getElementById('<%=txtagency_ACcode.ClientID%>').value;
                var txtadd = document.getElementById('<%=txtadd.ClientID%>').value;
                var ddlzone = document.getElementById('<%=ddlzone.ClientID%>');
                var ddlstate = document.getElementById('<%=ddlstate.ClientID%>');
                var ddlcity = document.getElementById('<%=ddlcity.ClientID%>');
                var txtpin = document.getElementById('<%=txtpin.ClientID%>').value;
                var txtdelivery_add = document.getElementById('<%=txtdelivery_add.ClientID%>').value;
                var ddldelivery_zone = document.getElementById('<%=ddldelivery_zone.ClientID%>');
                var ddldelivery_state = document.getElementById('<%=ddldelivery_state.ClientID%>');
                var ddldelivery_city = document.getElementById('<%=ddldelivery_city.ClientID%>');

                var txtcomission = document.getElementById('<%=txtcomission.ClientID%>').value;
                var txtorgcommision = document.getElementById('<%=txtorgcommision.ClientID%>').value;
                var ddlroute = document.getElementById('<%=ddlroute.ClientID%>');
                var txtpanchjanya_copies = document.getElementById('<%=txtpanchjanya_copies.ClientID%>').value;
                var txtorganiser_copies = document.getElementById('<%=txtorganiser_copies.ClientID%>').value;
                var txtsecurity = document.getElementById('<%=txtsecurity.ClientID%>').value;
                var txtsecureorg = document.getElementById('<%=txtsecureorg.ClientID%>').value;
                var ddldispatch = document.getElementById('<%=ddldispatch.ClientID%>');








                var error = "";
                var valid = true;

                if (txtdate == "") {
                    error += "Please Enter Date.\n";
                    valid = false;
                }
                else {
                    valid = true;
                }
                if (txtagency_name == "") {
                    error += "Please Enter Agency Name.\n";
                    valid = false;
                }
                else {
                    valid = true;
                }
                if (txtagency_code == "") {
                    error += "Please Enter Agency Code.\n";
                    valid = false;
                }
                else {
                    valid = true;
                }
                if (txtagency_ACcode == "") {
                    error += "Please Enter Agency Acc Code.\n";
                    valid = false;
                }
                else {
                    valid = true;
                }
                if (txtadd == "") {
                    error += "Please Enter Address.\n";
                    valid = false;
                }
                else {
                    valid = true;
                }

                if (ddlzone.selectedIndex == 0) {

                    error += "Please Select Zone.\n";
                    valid = false;

                }
                else {
                    valid = true;
                }
                if (ddlstate.selectedIndex == 0) {

                    error += "Please Select State.\n";
                    valid = false;

                }
                else {
                    valid = true;
                }
                if (ddlcity.selectedIndex == 0) {

                    error += "Please Select District.\n";
                    valid = false;
                }
                else {
                    valid = true;
                }


                if (txtdelivery_add == "") {
                    error += "Please Enter Delivery Address.\n";
                    valid = false;
                }
                else {
                    valid = true;
                }


                if (ddldelivery_zone.selectedIndex == 0) {

                    error += "Please Select Delivery Zone.\n";
                    valid = false;

                }
                else {
                    valid = true;
                }
                if (ddldelivery_state.selectedIndex == 0) {

                    error += "Please Select Delivery State.\n";
                    valid = false;

                }
                else {
                    valid = true;
                }
                if (ddldelivery_city.selectedIndex == 0) {

                    error += "Please Select Delivery District.\n";
                    valid = false;
                }
                else {
                    valid = true;
                }
                //            if (txtdelivery_pin == "") {
                //                error += "Please Enter Delivery Pin Code.\n";
                //                valid = false;
                //            }
                if (txtcomission == "") {
                    error += "Please Enter Panch Commission.\n";
                    valid = false;
                }
                else {
                    valid = true;
                }
                if (txtorgcommision == "") {
                    error += "Please Enter Org Commission.\n";
                    valid = false;
                }
                else {
                    valid = true;
                }
                if (ddlroute.selectedIndex == 0) {

                    error += "Please Select Route Name.\n";
                    valid = false;

                }
                else {
                    valid = true;
                }
                if (txtpanchjanya_copies == "") {
                    error += "Please Enter No of Panchjanya Copies.\n";
                    valid = false;
                }
                else {
                    valid = true;
                }
                if (txtorganiser_copies == "") {
                    error += "Please Enter No of Organiser Copies.\n";
                    valid = false;
                }
                else {
                    valid = true;
                }

                if (txtsecurity == "") {
                    error += "Please Enter Security Money.\n";
                    valid = false;
                }
                else {
                    valid = true;
                }
                if (txtsecureorg == "") {
                    error += "Please Fill Org Security.\n";
                    valid = false;

                }
                else {
                    valid = true;
                }
                if (ddldispatch.selectedIndex == 0) {

                    error += "Please Select Mode of Dispatch.\n";
                    valid = false;

                } else {
                    valid = true;
                }

                if (valid == false) {
                    setTimeout(function () {
                        alert(error);
                    }, 1000);
                }
                return valid;

            }


            function validateZone() {

                var txtZone = document.getElementById('<%=txtZone.ClientID%>').value;
                var error = "";
                var valid = true;

                if (txtZone == "") {
                    error += "Please Enter Zone Name.";
                    valid = false;
                }
                if (valid == false) {
                    alert(error);
                }
                return valid;

            }
            function validateState() {

                var txtState = document.getElementById('<%=txtState.ClientID%>').value;
                var error = "";
                var valid = true;

                if (txtState == "") {
                    error += "Please Enter State Name.";
                    valid = false;
                }
                if (valid == false) {
                    alert(error);
                }
                return valid;

            }


            function validateDistt() {

                var txtDistt = document.getElementById('<%=txtDistt.ClientID%>').value;
                var error = "";
                var valid = true;

                if (txtDistt == "") {
                    error += "Please Enter District Name.";
                    valid = false;
                }
                if (valid == false) {
                    alert(error);
                }
                return valid;

            }

            function validateRoute() {

                var txtRoute = document.getElementById('<%=txtRoute.ClientID%>').value;
                var error = "";
                var valid = true;

                if (txtRoute == "") {
                    error += "Please Enter Route Name.";
                    valid = false;
                }
                else {
                    valid = true;
                }

                if (valid == false) {
                    alert(error);
                }
                else {
                    valid = true;
                }
                return valid;

            }

</script>