Error "An unhandled exception occurred ... Value cannot be null"

ISSUE

When attempting to run a workflow that was imported from another list, the workflow suspends part way through and the following error occurs:

RESOLUTION

Within the actions inside of the workflow designer verify that all required fields still contain data. If any required fields are empty, re-enter the appropriate value into the field. 

 

The action with the field in question can often be determined by specifics at the end of the the error message returned from Workflow Manager.

ADDITIONAL INFORMATION

This error can also present as "An unhandled exception occurred during the execution of the workflow instance" with no null value error.  In this case check the stack trace in the internal status to verify which action type is causing the error and verify that the references for those actions are not returning null values.