How to reproduce the error:
Go to the edit patient form(either short or long), check on the deceased option, click on the 'change' button to the change the cause of death, add data to the field that is shown and click save.
You will realize that the error page is shown irrespective of whether you provide a change reason for cause of death or not.
Appropriate behavior should be sending the user back to the form and show the error message in case they haven't provided it or the field should be optional, and it also appears like the provided change reason for 'cause of death' is not being read and set by the controllers.
The fix probably affects a couple of earlier versions besides the one marked on this ticket.