Error while saving new book

Hi,

I’m sometimes getting an error when creating a new book. I press save after choosing an pdf and then I get the following error. If I delete and recreate the book its working again. Resaving another book that’s already working also causes the error sometimes.

AH01071: Got error ‘PHP message: PHP Fatal error: Uncaught TypeError: array_merge(): Argument #1 must be of type array, string given in?/wp-content/plugins/3d-flipbook-dflip-lite/inc/metaboxes.php:705\nStack trace:\n#0?/wp-content/plugins/3d-flipbook-dflip-lite/inc/metaboxes.php(705): array_merge()\n#1?/wp-includes/class-wp-hook.php(326): DFlip_Meta_boxes->save_meta_boxes()\n#2?/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters()\n#3?/wp-includes/plugin.php(517): WP_Hook->do_action()\n#4?/wp-includes/post.php(4828): do_action()\n#5?/wp-includes/post.php(4930): wp_insert_post()\n#6 /wp-admin/includes…’, referer: /wp-admin/post.php?post=229351&action=edit&classic-editor=1

All the best,
Patrick