I fixed it by reading json value which is generated and saved in hidden field
function OnClientDropDownOpening(sender, eventArgs) {
if ($("#RadComboBox1_ClientState").val() != "") {
var obj = jQuery.parseJSON($("#RadComboBox1_ClientState").val());
if (obj.text === "") {
sender.set_cancel(true);
}
} else {
eventArgs.set_cancel(true);
}
}