If you're getting an object reference not set to an instance of an object error when clicking on an item within the workflow then please follow the below steps:
Close and reopen DPS.
Sign out (Do not disconnect) and sign back in (If signed in on a server)
If you're not signed into a server, restart your machine.
If you're still receiving this error then please raise a new case online and reference the title of this article.
Please include the following in the ticket;
The steps you're taking to get this error
A screenshot of the error (Click more details on the error message).
When did this error start appearing?
Is this happening on all items on the workflow or specific items?
Are you just affected or are more users affected?