Account Types form; duplicating an existing Promotion may fail with an unhandled exception error

When duplicating an existing Account Type Promotion and then cancelling the process, the following error can occur "An unhandled exception has occurred. Index was outside the bounds of the array.".

Steps to recreate:

1. From the Admin menu, open Account Types.
2. Select an Account Type with an existing Promotion.
3. Click Edit.
4. On the Promotions page select a Promotion.
5. Click on the "Duplicate the current row" button.
6. Click "Cancel".
7. Click on the form's "Cancel Edit" button.