How to send data from ui page to ui action

[MarinaL]

Hi everyone!
I'm open popup from Ui action in CSM workspace this way:

Workspace client script:

g_modal.showFrame({
        url: url,
        title: 'Choose resolve method',
        size: 'sm',
        callback: function(response) {
            if(response){
                 alert(response.data); 
            }else{
                 alert('error');
            }
        }
    });

I want to receive data back from UI page i called using functionality:

 function validateSelect() {
        var select = document.getElementById('method');
        var message = document.getElementById('validationMessage');
        if (select.value === "") {
            message.style.display = 'block';
            message.innerHTML = "Please select an option.";
        } else {
            message.style.display = 'none';
            message.innerHTML = "";
            sendResponse({action: 'ok', data: select.value});
        }
    }

    function sendResponse(response) {
        window.parent.postMessage(response, "*");
    }
    </script>

I don't get anything. I want to fetch data from a UI page but am getting an error warning. Maybe someone knows what's wrong?

Thanks in advance!