Replace
//Add scripts to disable additional viewers:
$options = get_option( 'additional_viewers' );
if (1 == $options['fancybox']) {
by
//Add scripts to disable additional viewers:
$options = get_option( 'additional_viewers' );
if ($options && isset($options['fancybox']) && 1 == $options['fancybox']) {
worked for me
