Guest Checkout 'Print' feature doesn't work

For those of you after a solution to this issue, I’ve rewritten the create_print_button_order_page function in /includes/class-wcdn-theme.php.

/**
		 * Create a print button for the 'View Order' page
		 */
		public function create_print_button_order_page( $order_id ) {
			if( get_option( WooCommerce_Delivery_Notes::$plugin_prefix . 'print_button_on_view_order_page' ) ) {

				$print_url = '';
				/*
				 * Get the order details and check if we have an order key
				 */
				if(isset($_GET['key']))
				{
					$order_key 	= $_GET['key'];
					$order 		= new WC_Order( $order_id );			

					/*
					 * Check if our order key matches what is on file
					 */
					if($order->order_key == $order_key)
					{
						$email = $order->billing_email;
						$print_url = wcdn_get_print_link( $order_id, $this->template_type, $email );
					}

				}
				else
				{
					$print_url = wcdn_get_print_link( $order_id, $this->template_type );
				}

				if(strlen($print_url) > 0): ?>
					<p class="order-print">
						<a href="<?php echo $print_url; ?>" class="button print"><?php _e( 'Print', 'woocommerce-delivery-notes' ); ?></a>
					</p>
				<?php
				endif;
			}
		}