Warning: Illegal string offset 'id'

Hi,
we have update my meta box group to last version and now show this:

Warning: Illegal string offset 'id' in wp-content/plugins/meta-box-group/group-field.php on line 91

whenever I have the group boxes

how can i solve?

Tnk

Example code:

<div>
'fields' => array(
        // Group
        array(
                'id' => "{$prefix}Cart",
                // Group field
                'type'   => 'group',
                // Clone whole group?
                'clone'  => true,
                'sort_clone'=>true,
                'max_clone' => 4,
                'fields' => array(
                array(
                            'name'        => "Articolo",
                            'id'          => "art_id", 
                            'type'        => 'post',                             
                            // Post type: string (for single post type) or array (for multiple post types)
                            'post_type'   => array('post','glossario','focus'),
                            // Default selected value (post ID)
                            'std'         => 1,
                            // Field type, either 'select' or 'select_advanced' (default)
                            'field_type'  => 'select_advanced',
                            'desc' => 'Può essere: articolo, focus e glossario.' ,
                            // Placeholder
                            'placeholder' => __( 'Seleziona articolo', 'your-prefix' ),
                            'js_options'      => array(
                                                            'width' => '95%',
                                                        ),
                            // Query arguments (optional). No settings means get all published posts
                            // @see https://codex.wordpress.org/Class_Reference/WP_Query
                            'query_args'  => array(
                                'post_status'    => 'publish',
                                'posts_per_page' => - 1,
                            ),
                        ), ) )</div>