Is it that specific widget that doesn't show, or any widget?
Actually there's different widgets that are no longer working.
What happens when you put it in another widget spot of the theme?
Same problem. Different place.
What happens when you add a third widget to the sidebar?
Same problem--no show.
What happens if you use another theme?
If I switch to the Twenty Seventeen theme, then there's no problem. I think the big problem is that after the WP 4.9 update, a lot of things quit working (I've got two other plugins that took a dump after updating WP). I'm thinking the theme that is being used also has issues with that WP update as well and everything is rather turned upside down.
I've read on WP support that 4.9 has caused a lot of issues. They claim (as of a few hours before this post) that they will be coming out with a fix but no timeframe