json - Form success message not showing on successful submission with PHP - php

Question

I have difficulty in showing success message on form. Its showing all the error messages but not the success message.
On controller file its showing when I comment $this->model_catalog_outofstockquery->addQuery($this->request->post);
but this line is necessary.
On submitting form the entries are successfully loading to the database.
I have already call model from the constructor
My code on controller file is-
public function write() {
$this->load->language('product/outofstock_enquiry');
$json = array();
if ($this->request->server['REQUEST_METHOD'] == 'POST') {
if ((utf8_strlen($this->request->post['name']) < 3) || (utf8_strlen($this->request->post['name']) > 25)) {
$json['error'] = $this->language->get('error_name');
}
if (!filter_var($this->request->post['email'], FILTER_VALIDATE_EMAIL)) {
$json['error'] = $this->language->get('error_email');
}
if ((!filter_var($this->request->post['quantity'],FILTER_VALIDATE_INT))) {
$json['error'] = $this->language->get('error_quantity');
}
if (!isset($json['error'])) {
$json['success'] = $this->language->get('text_success');
$data['product_id'] = $this->request->post['product_id'];
$data['product_name'] = $this->request->post['product_name'];
$data['name'] = $this->request->post['name'];
$data['email'] = $this->request->post['email'];
$data['quantity'] = $this->request->post['quantity'];
$data['notify'] = 0;
$this->model_catalog_outofstockquery->addQuery($this->request->post);
}
}
$this->response->addHeader('Content-Type: application/json');
$this->response->setOutput(json_encode($json));
}
The script on the view file under the form is
<script type="text/javascript"><!--
$('#button-submit').on('click', function() {
$.ajax({
url: 'index.php?route=product/outofstock_enquiry/write&product_id=<?php echo $product_id; ?>',
type: 'post',
dataType: 'json',
data: 'name=' + encodeURIComponent($('input[name=\'name\']').val()) + '&email=' + encodeURIComponent($('input[name=\'email\']').val()) + '&product_name=' + encodeURIComponent($('input[name=\'product_name\']').val()) + '&quantity=' + encodeURIComponent($('input[name=\'quantity\']').val()) ,
data: $("#form-ask").serialize(),
beforeSend: function() {
$('#button-submit').button('loading');
},
complete: function() {
$('#button-submit').button('reset');
},
success: function(json) {
$('.alert-success, .alert-danger').remove();
if (json['error']) {
$('#outofstock_enquiry').after('<div class="alert alert-danger"><i class="fa fa-exclamation-circle"></i> ' + json['error'] + '</div>');
}
if (json['success']) {
$('#outofstock_enquiry').after('<div class="alert alert-success"><i class="fa fa-check-circle"></i> ' + json['success'] + '</div>');
$('input[name=\'name\']').val('');
$('input[name=\'email\']').val('');
$('input[name=\'product_name\']').val('');
$('input[name=\'quantity\']').val('');
$('input[name=\'product_id\']').val('');
}
}
});
});
//--></script>