您好尝试使用MySQL数据库作为示例进行基本CRUD操作，并且该平台中的新功能遇到了一些问题，请帮助
A PHP Error was encountered
Severity: Notice
Message: Undefined property: Products::$db
Filename: core/Model.php
Line Number: 73
Backtrace:
File: C:\xampp\htdocs\CodeIgniter-3.1.9\application\models\ProductsModel.php
Line: 9
Function: __get
File: C:\xampp\htdocs\CodeIgniter-3.1.9\application\controllers\Products.php
Line: 17
Function: get_products
File: C:\xampp\htdocs\CodeIgniter-3.1.9\index.php
Line: 315
Function: require_once
An uncaught Exception was encountered
Type: Error
Message: Call to a member function get() on null
Filename: C:\xampp\htdocs\CodeIgniter-3.1.9\application\models\ProductsModel.php
Line Number: 9
Backtrace:
File: C:\xampp\htdocs\CodeIgniter-3.1.9\application\controllers\Products.php
Line: 17
Function: get_products
File: C:\xampp\htdocs\CodeIgniter-3.1.9\index.php
Line: 315
Function: require_once