Unable to Insert Gallery

When I click on “Select Gallery” button on Responsive Gallery Block in backend it just doesn’t work and I get this warning:

WordPress version 5.3.2
Current theme: Twenty Twenty (version 1.1)
Current plugin: Responsive Lightbox & Gallery (version 2.2.2)
PHP version 7.3.10

Error Details
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An error of type E_ERROR was caused in line 503 of the file /web/htdocs/staging.mywebsite.com/home/wp-content/plugins/responsive-lightbox/includes/class-frontend.php. Error message: Uncaught Error: Unsupported operand types in /web/htdocs/staging.mywebsite.com/home/wp-content/plugins/responsive-lightbox/includes/class-frontend.php:503
Stack trace:
#0 /web/htdocs/staging.mywebsite.com/home/wp-content/plugins/responsive-lightbox/includes/class-galleries.php(3677): Responsive_Lightbox_Frontend->get_unique_fields(Array, NULL)
#1 /web/htdocs/staging.mywebsite.com/home/wp-content/plugins/responsive-lightbox/includes/class-galleries.php(4023): Responsive_Lightbox_Galleries->save_gallery(Array, 518, true)
#2 /web/htdocs/staging.mywebsite.com/home/wp-includes/class-wp-hook.php(288): Responsive_Lightbox_Galleries->save_revision(518)
#3 /web/htdocs/staging.mywebsite.com/home/wp-includes/class-wp-hook.php(312): WP_Hook->apply_filters(”, Array)
#4 /web/htdocs/staging.mywebsite.com/home/wp-includes/plugin.php(478): WP_Hook->do_action(Array)
#5 /web/htdocs/staging.mywebsite.com/home/wp-includes/revision.php(329): do_action(‘_wp_put_post_re…’, 518)
#6 /web/htdocs/staging.mywebsite.com