Warning for normalize_whitespace

When I want to restore a revision in Wordpress, I got the following warning:

Warning: trim() expects parameter 1 to be string, array given in ./wp-includes/formatting.php on line 4217

The problem is that the plugin Advanced-Custom-Fields handles an array in the variable $post->news-content.

news-content is a custom field property with a flexible content type. I didn't test other types yet.

My question: Where is the mistake?

Must ACF store the content as serialized data in $page->fields or must Wordpress Core change the compare block in wp_save_post_revision (line 157ff) for more compatibility?

via Chebli Mohamed