Opencart 2 Add custom customer data to admin customer profile page

Question

I have a new mysql table I call co_customer that stores additional unique content about a customer. This content I have all viewable and working on the customer registration form. However I am now trying to make these new custom fields editable on the admin panel customer profile page. I am currently just testing this our to get one of the new fields to work. However the new field is blank and I am getting the below error at the top of the page

Notice: Undefined index: cofirstname in  
/home/public_html/admin/controller/customer/customer.php on line 972

I edited the below file public_html/admin/model/customer/customer.php

public function getCustomer($customer_id) 
 {
     $query = $this->db->query("SELECT DISTINCT * FROM " . DB_PREFIX . "customer WHERE customer_id = '" . (int)$customer_id . "'");
     return $query->row;

             //get coapplicant data
             $cocustomersql = $this->db->query("SELECT * FROM " . DB_PREFIX . "co_customer WHERE customer_id = '" . (int)$customer_id . "'");
             if ($cocustomersql->num_rows > 0) 
             {
                  return $cocustomersql->row;
             }
 }

I edited the below file public_html/backoffice/controller/customer/customer.php

 if (isset($this->request->post['cofirstname'])) 
    {
        $data['cofirstname'] = $this->request->post['cofirstname'];
    } elseif (!empty($customer_info)) {
        $data['cofirstname'] = $customer_info['cofirstname'];
    } else {
        $data['cofirstname'] = '';
    }

Answer 1

I suggest use the existing feature of OpenCart rather than editing code.

Login Admin/Customers/Custom Fields , here you can add fields and assign it to customer

for detail