Woo commerce - Uncaught Error：在null上调用成员函数get（）

Question

I am creating an extension plugin for WooCommerce and am getting the error Uncaught Error: Call to a member function get() on null

The code that is causing this is:

foreach ( $this->cart_session_data as $key => $default ) {
                $this->$key = WC()->session->get( $key, $default );
            }

The full code can be seen here http://pastebin.com/v9e2QrLv

Thanks in advance for your help