Add appointment to order from Dashboard error

[Valentina R]

When creating a new appointment from the backend and selecting to add it to an existing order, it throws warnings and notices and doesn't create the item. But it adds the price.

Reported here: https://bizzthemes.com/forums/topic/adding-appointment-from-the-backend-gives-a-error/

[Željan Topić]

Fixed with 3.6.9