Advanced Custom Fields: Extended - Fatal error: Uncaught InvalidArgumentException: item should be a Collection or an array or an object in...

Overview of the issue

You may experience a fatal error after updating the Advanced Custom Fields Multilingual plugin to version 2.1.4.1 when also using the Advanced Custom Fields: Extended and Advanced Custom Fields Pro plugins:

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 …/wp-content/plugins/acfml/classes/class-wpml-acf-options-page.php file.
• Look for line 280.
• Replace:

  	private static function isValidOptionPagePostId( $postId ) {
  		return function_exists( 'acf_get_options_pages' )
  		       && wpml_collect( acf_get_options_pages() )->first( Relation::propEq( 'post_id', $postId ) );
  	}
  

• With:

  	private static function isValidOptionPagePostId( $postId ) {
  		if ( function_exists( 'acf_get_options_pages' ) ) {
  			$optionPages = acf_get_options_pages();
  		
  			if ( $optionPages ) {
  				return wpml_collect( acf_get_options_pages() )->first( Relation::propEq( 'post_id', $postId ) );
  			}		
  		}
  	
  		return false;
  	}