Undefined property

I think the program is correct, but it appears that it is an error. Please someone help me.

A PHP Error was encountered
Severity: Notice
Message: Undefined property: mysqli::$ProductID
Filename: controllers/ProductsUI.php


CI_Model:

PHP Code:

function get_product(){
       $this->load->database();
       return $this->db->get('product');
   } 


CI_Controller:

PHP Code:

$str = '';
        $this->load->model('Product');
        foreach($this->Product->get_product() as $row){
            $str += 'tr role="row">
            <td>'.$row->ProductID.'</td>
            <td>'.$row->Product_Name.'</td>
            <td>'.$row->Cost.'</td>
            <td><button>แก้ไข</button></td>
            <td><button>ลบ</button></td>
            </tr>';
        }