I want to make simple template rendering in Slim3 but I get an error:

Here is my code :
namespace controller;

```
class Hello
{
    function __construct() {
// Instantiate the app
        $settings = require __DIR__ . '/../../src/settings.php';
        $this->app = new \Slim\App($settings);
    }

    public function index(){
        return $this->app->render('web/pages/hello.phtml');   //LINE20
    }
}

```

This is the error I get : 

```
Message: Method render is not a valid method

```