After updating to 5.3.6 I get this warning message on all nested flexible content rows. This field is a flex field nested in a repeater, nested in a flex field.
Warning: array_pop() expects parameter 1 to be array, null given in /srv/www/example.com/current/web/app/plugins/advanced-custom-fields-pro/pro/fields/flexible-content.php on line 1261
NP, just wanted to let you know so you weren’t waiting.