Undefined notice on unset field

The error is that you’re trying to read $value['kk_youtube'] when there is no kk_youtube key in the array (undefined index). Rather than use isset, use get_post_meta with the third argument “single” true:

$youtube_link = esc_attr( get_post_meta( $post->ID, 'kk_youtube', true ) );

This is effectively the same as doing:

$value = get_post_custom( $post->ID );
if ( isset( $value['kk_youtube'][0] ) )
     $youtube_link = esc_attr( $value['kk_youtube'][0] );
else
     $youtube_link = '';