Error after installation

Hi there!

Looks to be a useful plugin for my case. Thanks so much!
I installed the plugin and when I save/publish the gallery, it gives this error page. I installed it on LocalWP dev site and I’m working with WordPress 5.5 Gutenberg. Could you please let me know what’s wrong?

( ! ) Warning: Illegal string offset ‘option_type’ in /Users/rupaamna/Local Sites/infinity-pro-repo/app/public/wp-content/plugins/feed-them-gallery/metabox-settings/metabox-settings-class.php on line 848
Call Stack
# Time Memory Function Location
1 0.0002 410792 {main}( ) …/post.php:0
2 0.1946 3447856 edit_post( ) …/post.php:227
3 0.2218 3520016 wp_update_post( ) …/post.php:419
4 0.2229 3547912 wp_insert_post( ) …/post.php:4354
5 0.2365 3597224 do_action( ) …/post.php:4234
6 0.2365 3597600 WP_Hook->do_action( ) …/plugin.php:478
7 0.2365 3597600 WP_Hook->apply_filters( ) …/class-wp-hook.php:311
8 0.2366 3598728 wp_save_post_revision( ) …/class-wp-hook.php:289
9 0.2380 3599240 _wp_put_post_revision( ) …/revision.php:190
10 0.2382 3602096 wp_insert_post( ) …/revision.php:332
11 0.2504 3612624 do_action( ) …/post.php:4260
12 0.2504 3613000 WP_Hook->do_action( ) …/plugin.php:478
13 0.2504 3613000 WP_Hook->apply_filters( ) …/class-wp-hook.php:311
14 0.2505 3614160 feed_them_gallery\Metabox_Settings->save_meta_box( ) …/class-wp-hook.php:289

( ! ) Warning: Illegal string offset ‘option_type’ in /Users/rupaamna/Local Sites/infinity-pro-repo/app/public/wp-content/plugins/feed-them-gallery/metabox-settings/metabox-settings-class.php on line 848
Call Stack
# Time Memory Function Location
1 0.0002 410792 {main}( ) …/post.php:0
2 0.1946 3447856 edit_post( ) …/post.php:227
3 0.2218 3520016 wp_update_post( ) …/post.php:419
4 0.2229 3547912 wp_insert_post( ) …/post.php:4354
5 0.2786 3603376 do_action( ) …/post.php:4260
6 0.2786 3603752 WP_Hook->do_action( ) …/plugin.php:478
7 0.2786 3603752 WP_Hook->apply_filters( ) …/class-wp-hook.php:311
8 0.2787 3604160 feed_them_gallery\Metabox_Settings->save_meta_box( ) …/class-wp-hook.php:289

( ! ) Warning: Cannot modify header information – headers already sent by (output started at /Users/rupaamna/Local Sites/infinity-pro-repo/app/public/wp-content/plugins/feed-them-gallery/metabox-settings/metabox-settings-class.php:848) in /Users/rupaamna/Local Sites/infinity-pro-repo/app/public/wp-includes/pluggable.php on line 1296
Call Stack
# Time Memory Function Location
1 0.0002 410792 {main}( ) …/post.php:0
2 0.2922 3552976 redirect_post( ) …/post.php:234
3 0.2934 3553072 wp_redirect( ) …/post.php:2034
4 0.2935 3553160 header ( ) …/pluggable.php:1296

( ! ) Warning: Cannot modify header information – headers already sent by (output started at /Users/rupaamna/Local Sites/infinity-pro-repo/app/public/wp-content/plugins/feed-them-gallery/metabox-settings/metabox-settings-class.php:848) in /Users/rupaamna/Local Sites/infinity-pro-repo/app/public/wp-includes/pluggable.php on line 1299
Call Stack
# Time Memory Function Location
1 0.0002 410792 {main}( ) …/post.php:0
2 0.2922 3552976 redirect_post( ) …/post.php:234
3 0.2934 3553072 wp_redirect( ) …/post.php:2034
4 0.2959 3553216 header ( ) …/pluggable.php:1299