ISSUEWhen 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 CODEThe ULS logs also captures the error with the following entry:
RESOLUTIONRepublish the workflow with an existing, valid user account.
ADDITIONAL INFORMATIONThe 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).