erron in ultimate-wpqsf.php error line 129

Hello all,
I have a error in the plugins Ultimate WP Query Search Filter, I see this error when I try to save a new filter in the Ultimete plugin.

Fatal error: Uncaught Error: [] operator not supported for strings in /home/classicmotorclub/.ftp-users/wp-content_lzDD/plugins/ultimate-wp-query-search-filter/ultimate-wpqsf.php:129 Stack trace: #0 /home/classicmotorclub/www/wp-includes/class-wp-hook.php(307): ulitmatewpsf->uwpqsf_save_from('') #1 /home/classicmotorclub/www/wp-includes/class-wp-hook.php(331): WP_Hook->apply_filters(NULL, Array) #2 /home/classicmotorclub/www/wp-includes/plugin.php(476): WP_Hook->do_action(Array) #3 /home/classicmotorclub/www/wp-admin/admin.php(175): do_action('admin_init') #4 {main} thrown in /home/classicmotorclub/.ftp-users/wp-content_lzDD/plugins/ultimate-wp-query-search-filter/ultimate-wpqsf.php on line 129

Any idea? Thanks