Passing GET value show error

That seems a fair approach but you could use site_url instead, just a bit cleaner.

PHP Code:

<a href='<?php echo site_url('documentazione/crea_documentazione_seconda/'.$row->id); ?>' class='btn btn-primary btn-xs'> 


If pressing that button is causing the missing argument, then just take a look at the html generated for the button and check the id is being found and is populated as you expected.

Other than that I cannot debug your code, sorry.

Paul

No with a pression of button the sistem work fine , the problem is when i submit the form without passing the form_validation and an argoment missing when loading  $this->load->view('documentazione/crea_documentazione_seconda')