That's because you apparently have a user without any address.
Just change the line:
<?php $html = $this->cart->displayButton(JText::_('HIKA_NEW'),'new',$this->params,'','var link = document.getElementById(\'hikashop_checkout_'. $this->type.'_address_new_link\'); if(link) return hikashopEditAddress(link,'.(int)$same_address.',true); return false;');
to:
<?php $html = $this->cart->displayButton(JText::_('HIKA_NEW'),'new',$this->params,'','var link = document.getElementById(\'hikashop_checkout_'. $this->type.'_address_new_link\'); if(link) return hikashopEditAddress(link,'.(int)@$same_address.',true); return false;');
in that file via FTP and the message won't be displayed anymore.