$mailvars_function can be null and cause cron failure

I was getting this error in cron:

Fatal error: Function name must be a string in /sites/all/modules/subscriptions/subscriptions_mail.module on line 121

I tracked it down and realized that for whatever reason $mailvars_function wasn't getting a value and there was no catch for it or default value. I didn't spend a lot of time figuring out how to reproduce this issue or digging deeper to figure out why that variable wasn't available, but all the basic checks were done and made sure all the settings were saved, etc. So, through the docs i found a value that should work (hopefully its a reasonable default) and made this fix.

          $mailvars_function = $fields[$locale][$module][$field]['mailvars_function'];
          watchdog('debug', '#1: $mailvars_function is '. (isset( $mailvars_function ) ? 'set' : 'null'));
          if( is_null($mailvars_function) ) {
            $mailvars_function = '_subscriptions_content_node_mailvars';
          }

Interested to find out more about to find out more about this, but it was a last minute bug that was discovered after going live with a site so will post back if there is additional bad behaviour or info.