Advanced Custom Fields - Fatal error: Uncaught InvalidArgumentException: item should be a Collection...

Overview of the issue

A critical issue has been identified in Advanced Custom Fields Multilingual 2.1.0 where a fatal error may appear when Advanced Custom Fields forms are displayed on the frontend:

Fatal error: Uncaught InvalidArgumentException: item should be a Collection or an array or an object in .../wp-content/plugins/sitepress-multilingual-cms/vendor/wpml/fp/core/Obj.php:112.

Workaround

Please, make sure of having a full site backup of your site before proceeding.

• Open your wp-config.php file.
• Add this line

  define( 'ACFML_HIDE_FIELD_ANNOTATIONS', true );

  before:

  /* That's all, stop editing! Happy publishing. */