WordPress version 5.7 compatibility

Hi, this should be quite easy to fix and might resolve the WP 5.7 issues:

When seeking help with this issue, you may be asked for some of the following information:
WordPress version 5.7
Current plugin: Sync Post With Other Site (version 1.2.3)
PHP version 7.4.3

Error Details
=============
An error of type E_ERROR was caused in line 118 of the file .../wp-content/plugins/sync-post-with-other-site/includes/sps_sync.class.php. Error message: Uncaught Error: Cannot use object of type WP_Error as array in .../wp-content/plugins/sync-post-with-other-site/includes/sps_sync.class.php:118
Stack trace:
#0 .../wp-includes/class-wp-hook.php(292): SPS_Sync->sps_save_post()
#1 .../wp-includes/class-wp-hook.php(316): WP_Hook->apply_filters()
#2 .../wp-includes/plugin.php(484): WP_Hook->do_action()
#3 .../wp-includes/post.php(4384): do_action()
#4 .../wp-includes/post.php(4486): wp_insert_post()
#5 .../wp-admin/includes/post.php(419): wp_update_post()
#6 .../wp-admin/post.php(227): edit_post()
#7 {main}
 thrown