For guest user why are you trying to save organisation id in customer profile, there will no customer profile to save because Magento doesn't create customer in customer entity for guest users. You need to add condition to your function above to resolve the problem
if(!$order->getCustomerIsGuest()){
// Customer stuff
$this->_customer_id = $this->_order->getCustomerId();
$this->_customer = $this->_order->getCustomer();
// problem on the next line below #PROBLEM HERE#
$this->_customer->setOrganisationId($organisation_id)->save();
}
Hope the above helps
Cheers S