Gravity Forms User Registration Add-On - PHP Fatal error: Uncaught Error: Class 'GFSignup' not found

Overview of the issue

If you are using the Gravity Forms User Registration Add-On and you try to add a new WordPress user, you will get this error:

PHP Fatal error: Uncaught Error: Class ‘GFSignup’ not found in /wp-content/plugins/gravityforms-multilingual/classes/compatibility/user_registration/Hooks.php:46

Workaround

Before proceeding make a full backup of your site.

1. Open /wp-content/plugins/gravityforms-multilingual/classes/compatibility/user_registration/Hooks.php file.
2. Look for line 42.
3. Replace:

       public function onActivation( $meta, WP_User $user, $update ) {
       	if ( ! $update ) {
       		$key    = rgpost( 'key' ); // From ajax.
       		$key    = $key ?: rgpost( 'item' ); // From form submission.
       		$signup = GFSignup::get( $key );
       		if ( $signup instanceof GFSignup && $signup->meta['email'] === $user->user_email ) {
       			$meta['icl_admin_language'] = $signup->meta['icl_admin_language'];
       			$meta['locale']             = $signup->meta['icl_admin_locale'];
       		}
       	}
       	return $meta;
       }
   

4. With:

       public function onActivation( $meta, WP_User $user, $update ) {
       	if ( ! $update && class_exists( 'GFSignup' ) ) {
       		$key    = rgpost( 'key' ); // From ajax.
       		$key    = $key ?: rgpost( 'item' ); // From form submission.
       		$signup = GFSignup::get( $key );
       		if ( $signup instanceof GFSignup && $signup->meta['email'] === $user->user_email ) {
       			$meta['icl_admin_language'] = $signup->meta['icl_admin_language'];
       			$meta['locale']             = $signup->meta['icl_admin_locale'];
       		}
       	}
       	return $meta;
       }