Fix for woocommerce-xero extension. Original plugin depends on billing address. In our case, we have disabled billing/shipping address so first_name and last_name should be fetched from WordPress usermeta.

woocommerce-xero.php

                        // construct XML
                        $xml = ''; 
                    $xml .= '<Invoices>';
                    $xml .= '<Invoice>';
                    $xml .= '<Type>ACCREC</Type>';
                    $xml .= '<Contact>';

                        if( $order->billing_first_name == ""){
                                $order->billing_first_name  =  get_usermeta($order->user_id,'first_name'); 
                        }   
                         if( $order->billing_last_name == ""){
                                 $order->billing_last_name  =  get_usermeta($order->user_id,'last_name');
                         }   


                    // Company name will be used as main name if it exists
                    $xml .= '<InvoiceNumber>WC-' . ltrim( $order->get_order_number(), _x( '#', 'hash befor    e order number', 'woocommerce' ) ) . '</InvoiceNumber>';