When registering for an event using the Event Registration Form (Classic) part using a payment page with a payment 2.0 part, the below error is generated after completing payment. Note: The error might display as either just "Value cannot be null. Parameter name: source" or the full stack trace.
In the instance reported, the error was caused by the use of the "Full Event Details" merge field on the Event Registration Form (classic) part. Removing the merge field fixed the issue on the payment page's confirmation screen.
Steps to Duplicate
1. Create or use an event registration form (classic) that utilizes a payment 2.0 part. 2. Register for the event for a single registrant. 3. Proceed to payment and pay. 4. Observer runtime error when completing payment.