Matt J. It's scary haha... I really didn't want to lose the progress
UX-wise is OKAY, I mean... There is a notification floating there.
Perhaps a popup, before the user goes too deep?
"Hey, you're not logged in... Are you sure you want to continue work on this thing, mate?
[Oh, good call! Thanks, I'll log in now]
[Yes, leave me alone and don't send me messages]