This occasionally happens when I use a post kind that requires a URL. I am using the native WordPress editor.
[Wed Jul 18 07:03:34.179394 2018] [:error] [pid 17074] [client 69.142.91.253:45190] PHP Warning: array_key_exists() expects parameter 2 to be array, null given in /var/www/html/wp-content/plugins/indieweb-post-kinds/includes/class-parse-mf2.php on line 249, referer: http://142.93.124.147/wp-admin/post-new.php
[Wed Jul 18 07:03:34.179463 2018] [:error] [pid 17074] [client 69.142.91.253:45190] PHP Warning: array_key_exists() expects parameter 2 to be array, null given in /var/www/html/wp-content/plugins/indieweb-post-kinds/includes/class-parse-mf2.php on line 249, referer: http://142.93.124.147/wp-admin/post-new.php
[Wed Jul 18 07:03:34.179477 2018] [:error] [pid 17074] [client 69.142.91.253:45190] PHP Warning: Invalid argument supplied for foreach() in /var/www/html/wp-content/plugins/indieweb-post-kinds/includes/class-parse-mf2.php on line 871, referer: http://142.93.124.147/wp-admin/post-new.php