Fix so you can preview ACF field changes in Wordpress admin

functions.php

<?php
/*
   Debug preview with custom fields
   Taken from: http://support.advancedcustomfields.com/forums/topic/preview-solution/
   See also: http://support.advancedcustomfields.com/forums/topic/2nd-every-other-post-preview-throws-notice/
*/
add_filter('_wp_post_revision_fields', 'add_field_debug_preview');
function add_field_debug_preview($fields){
   $fields["debug_preview"] = "debug_preview";
   return $fields;
}
add_action( 'edit_form_after_title', 'add_input_debug_preview' );
function add_input_debug_preview() {
   echo '<input type="hidden" name="debug_preview" value="debug_preview">';
}