The answer ended up being simple, although I'm not sure how 'correct it is:

Text(userEmail != null ? userEmail! : 'No Session Info'),
...
Text(userId != null ? '${userId!.substring(0,16)}...' : 'No Session Info'),
...

Simply adding a null check on the two textfields resolved the issue; now when the user clicks Delete Everything, their account and all associated data is deleted, they are logged out globally, and the UI pops back to the login/new user view automagically.