WordPress 4.9 compatibility

Plugin claims to be compatible with WordPress 4.7, but it looks like you’re using functions from WordPress 5+ and it throws an error in WordPress 4.9. This also makes the plugin incompatible with ClassicPress.

Specifically wp_is_json_request and wp_timezone_string.

You can test it using WPSeek’s Plugin Doctor.

I would recommend either bumping the minimum required version or using function_exists to check if those functions exist to continue to be compatible with WordPress 4.7 or 4.9 (and ClassicPress for that matter).