Fatal Error with Install

Hi,

I got a fatal error when I installed Swiss QR Bill on our website,

Fatal error: Uncaught Error: Call to a member function get() on null in <site>/plugins/swiss-qr-bill/includes/gateway/class-wc-gateway-swiss-qr-bill.php:286 Stack trace: 

#0 <site>/wp-content/plugins/swiss-qr-bill/includes/gateway/class-wc-gateway-swiss-qr-bill.php(145): WC_Gateway_Swiss_Qr->is_valid_billing_country() 

#1 <site>/wp-content/plugins/woocommerce/includes/class-wc-payment-gateways.php(151): WC_Gateway_Swiss_Qr->is_available() 

#2 <site>/wp-content/plugins/woocommerce/includes/wc-account-functions.php(118): WC_Payment_Gateways->get_available_payment_gateways() 

#3 <site>/wp-content/plugins/woo-custom-my-account-page/includes/class-woo-custom-my-account-page-functions.php(282): wc_get_account_menu_items() 

#4 <site>/wp-content/plugins/woo-custom-my-account-page/includes/class in <site>/wp-content/plugins/swiss-qr-bill/includes/gateway/class-wc-gateway-swiss-qr-bill.php on line 286

It talks about the following code:

    /**
     * Check if the customer if checking out from valid country
     * @return bool
     */
    public function is_valid_billing_country() {
        return in_array(strtoupper(WC()->session->get('customer')['country']), array('CH', 'LI'));
    }

Hope you can help us out soon! Thanks