how check response?

dhxFormAboutusAdd.send(“aboutusadd.php”, “post”,function(loader, response){
dhtmlx.message.defPosition = “middle”;
if(response == null){
dhtmlx.message({
text: “Saved successfully.”,
lifetime: 1000,
type: “message”
});
dhxFormAboutusAdd.clear();
editor.setData( ‘’ );
} else {
dhtmlx.alert(“Error. Try again!”);
//alert(response);
}
});