FREEMINUS ERROR

Hi,
I have this error coming up and would like to know how I can fix it, ideally step by step guide as I am no expert. Any help greatly appreciated.

Notice: Undefined index: is_connected in /home/gardene/cheltbeerfestival.co.uk/wp-content/plugins/tickera/freemius/includes/class-freemius.php on line 13889

Warning: Cannot modify header information – headers already sent by (output started at /home/gardene/cheltbeerfestival.co.uk/wp-content/plugins/tickera/freemius/includes/class-freemius.php:13889) in /home/gardene/cheltbeerfestival.co.uk/wp-admin/includes/misc.php on line 1310

Warning: Cannot modify header information – headers already sent by (output started at /home/gardene/cheltbeerfestival.co.uk/wp-content/plugins/tickera/freemius/includes/class-freemius.php:13889) in /home/gardene/cheltbeerfestival.co.uk/wp-admin/admin-header.php on line 9