ISSUE
When trying to publish a workflow that was previously designed and published with a user account that no longer exists, the workflow designer page displays a red banner with the following message:
Error. Object not set to an instance of an object.
Additionally, the workflow versions are not available.
ERROR CODE
The ULS logs also captures the error with the following entry: w3wp.exe (0x2E04) 0x1054 Nintex Workflow 2010 General 0000 Unexpected (/_layouts/NintexWorkflow/WorkflowDesigner.aspx?WorkflowId=3f6e925c-dd2f-4857-8180-7d7a1e8f577b&Category=List&ListId=35a80855-16d6-489c-a5fb-75ae4db4e83c): Nintex.Workflow.NWException: System.NullReferenceException: Object reference not set to an instance of an object. at Nintex.Workflow.ApplicationPages.WorkflowDesigner.Page_Load(Object sender, EventArgs e) (Build:25310 1c1e630c-f559-4b26-b031-791e9e6a0530
RESOLUTION
Republish the workflow with an existing, valid user account.
ADDITIONAL INFORMATION
The reason behind this error is the user/account that does not exist anymore in the Active Directory structure.
To minimize this problem as much as possible in the future consider using an account just for this task rather than a human user account (as people are likely to change often).
