I already have

the_widget(‘avia_combo_widget’, ‘error404widget’, array(‘widget_id’=>’arbitrary-instance-‘.$id,
‘before_widget’ => ‘<div class=”widget avia_combo_widget”>’,
‘after_widget’ => ‘</div>’,
‘before_title’ => ‘<h3 class=”widgettitle”>’,
‘after_title’ => ‘</h3>’
));

in the error404.php file!