Cannot access class properties from ajax call in wpordrpess

The problem was caused by another method inside the class:

     public function create_cart_id() {
        if( is_user_logged_in() ) {
            $this->create_logged_in_cart_id();
        }else {
            $this->create_non_logged_in_cart_id();
        }

        return $this->cart_id; - adding that line solved the problem
    }

This method generates the $cart_id, but it didn’t return its value, so it was not accessible from outside and returned null.