Hi Jeff,
The Users tab is the correct location to make these changes, you're running into a bug that we're working on addressing for our next release.
The issue is; you have a worker (or dispatcher) in your project whose named user was deleted from the Organization.
If you know which user was deleted; you can then simply delete the Worker record using the ArcGIS Online map viewer. If you don't know which user was deleted, let me know as I can provide the steps needed to identify which worker to remove.
Craig