How do I get the total amount from a column?

Hi,

I'm upgrading my CI3 model to CI4, but things are not working well.

CI 3

PHP Code:

// Sales
 $this->db->select_sum('total');
 $result = $this->x_model->get('order', array('created_on BETWEEN DATE_SUB(NOW(), INTERVAL 7 DAY) AND NOW()' => null));
 $data['sales_count'] = $result->total; 


CI 4

PHP Code:

  public function total_weekly_sales()
    {
        return $this->db->table('order')
        ->selectSum('total')
        ->where("created_on BETWEEN DATE_SUB(NOW(), INTERVAL 7 DAY) AND NOW()")
        ->get();
    } 


It doesn't work. It gives me

Code:

Object of class CodeIgniter\Database\MySQLi\Result could not be converted to string.

Any idea where am I doing wrong?