Change in code

[Valentina R]

Change $tabs['inventory']['class'] = array_push( $tabs['inventory']['class'], 'show_if_appointment' ); to $tabs['inventory']['class'][] = 'show_if_appointment';

Reported here: https://bizzthemes.com/forums/topic/yith-woocommerce-gift-cards/

[Željan Topić]

Fixed with 3.5.6