Unable to order multiple types of tickets at once

[mrsaicharan1]

Describe the bug
An order which is being placed with multiple types of tickets gives the following error:

  File "/Users/saicharan/Desktop/open-event-server/app/api/helpers/order.py", line 76, in create_pdf_tickets_for_holder
    order_ticket_info = OrderTicket.query.filter_by(order_id=order.id).one()
  File "/usr/local/lib/python3.7/site-packages/sqlalchemy/orm/query.py", line 3284, in one
    "Multiple rows were found for one()"
sqlalchemy.orm.exc.MultipleResultsFound: Multiple rows were found for one()

To Reproduce
Steps to reproduce the behavior:

1. Go to any event with paid & free tickets
2. Place an order with paid & free tickets
3. See error

Expected behavior
The order should go through

Additional context
Working on it.