Sometimes user gets deleted, e.g. in Admin Panel. If value in field stays the same, then PHP notice appears, stating:
[30-May-2018 18:25:01 Europe/London] PHP Notice: Trying to get property of non-object in acf\includes\fields\class-acf-field-user.php on line 567
The fix is quite simple: The $value in “format_value_single” function in class-acf-field-user.php has to be returned only when “$wp_user” finds an user with given id. If not, then probably empty array should be returned.
Please introduce a fix for this in next release. Or if it’s not possible, then please let me know how could I eventually fix that in a way to not modify core. Thank you.