Error when loading template.

The site I’m referencing is http://extended-replay.com/

When loading a template, I get the following error.

Warning: Creating default object from empty value in /home/www/extended-replay.com/wordpress/wp-admin/includes/post.php on line 632

Also tags, categories, etc. are all missing after loading the template, and instead of publishing, the only option is “Submit for Review”.

https://wordpress.org/plugins/simple-post-template/