uncaught exception: out of memory in Ajax Process

I got a problem I am submitting a simple form that has a small data and when I checked in the console tab the URL of ajax seems to be working but after the ajax is success it is redirected to my homepage and from the console tab I have this weird error:

uncaught exception: out of memory

In my ajax I have this simple code only:

$("#add-comment").on('click', function() {

    var id = $('input[name=\'review_id\']').val();
    var customer_id = $('input[name=\'customer_id\']').val();
    var $comment = $('textarea[name=\'user_comment\']').val();

    var sample = "test";

    $.ajax({
        url: 'index.php?route=product/product/writeSubComment',
        type: 'post',
        dataType: 'json',
        data: { text: sample },
        beforeSend: function() {

        },
        success: function(data) {
            console.log(data.test);
        },
        error: function() {
            alert('ERROR!!!');
        }
    });

});

In my PHP controller I have this function

public function writeSubComment() {

    $json = array();

    $json['test'] = $this->request->post['sample'];

    $this->response->addHeader('Content-Type: application/json');
    $this->response->setOutput(json_encode($json));

}