Michael J. Swart shows a scenario where the deadlock graph fails to open in SQL Server Management Studio:
I recently got this error in Management Studio when trying to view a deadlock graph that was collected with an extended events session:
Failed to initialize deadlock control.
Key cannot be null.
Parameter name: key
I found this error in a session that included the xml_deadlock_report event.
Read on for more information, and do check the comments where Lonny Niederstadt points out that even a victimless deadlocking scenario can have an ultimate victim: performance.