The Page adding with Organize Series is always break down

Hello, I have a problem with using Organize Series Plugin when adding some pages to a specific category.

I try tis page, but every time I open this page no matter which browser or in mobile, page adding with Organize Series always break down.

and the error monitor tell me this error happened.

PHP Fatal error: Uncaught Error: [] operator not supported for strings in /var/www/html/admin/wp-content/plugins/organize-series/orgSeries-template-tags.php:46

Stack trace:
#0 /var/www/html/admin/wp-content/plugins/organize-series/orgSeries-widgets.php(137): get_series_posts('', 'widget', false, 'Other posts in ...')
#1 /var/www/html/admin/wp-includes/class-wp-widget.php(372): orgSeries_widget_seriestoc->widget(Array, Array)
#2 /var/www/html/admin/wp-includes/widgets.php(742): WP_Widget->display_callback(Array, Array)
#3 /var/www/html/admin/wp-content/themes/ixion/sidebar.php(15): dynamic_sidebar('sidebar-1')
#4 /var/www/html/admin/wp-includes/template.php(688): require_once('/var/www/html/a...')
#5 /var/www/html/admin/wp-includes/template.php(647): load_template('/var/www/html/a...', true)
#6 /var/www/html/admin/wp-includes/general-template.php(110): locate_template(Array, true)
#7 /var/www/html/admin/wp-content/themes/ixion/single.php(77): get_sidebar()
#8 /var/www/html/admin/wp-includes/template-loader.php(74): include('/v in /var/www/html/admin/wp-content/plugins/organize-series/orgSeries-template-tags.php on line 46

I couldn’t figure out where is the conflict with my theme. Can you help me?
Thank you very much!

The page I need help with: [log in to see the link]