Uncaught TypeError in appointment reminder script

[mschoettle]

PHP Fatal error:  Uncaught TypeError: Cannot access offset of type string on string in /var/www/html/publisher/php/CustomPushNotification.php:55
Stack trace:
#0 /var/www/html/publisher/php/NextDayAppointmentNotification.php(55): CustomPushNotification::sendNotificationByPatientSerNum()
#1 /var/www/html/publisher/php/sendAppointmentReminders.php(11): NextDayAppointmentNotification::createAndSendNotifications()
#2 {main}
  thrown in /var/www/html/publisher/php/CustomPushNotification.php on line 55