Not sure you’re asking for error with Walker_Nav_Menu::start_el() or Walker_Nav_Menu::start_lvl(). But you must specify all arguments as default in the Walker_Nav_Menu class in your derived classes. E.g:
class description_walker extends Walker_Nav_Menu
{
/**
* Starts the list before the elements are added.
*
* @see Walker::start_lvl()
*/
public function start_lvl(&$output, $depth = 0, $args = array())
{
// your code...
}
/**
* Start the element output.
*
* @see Walker::start_el()
*/
public function start_el(&$output, $item, $depth = 0, $args = array(), $id = 0)
{
// your code...
}
}