It's a known bug. Some users have gotten an update to fix it. It's just a notification, the camera has already done its job. Just a strange notification bug. You'll see the update too.
Go into the Galaxy store and see if you still have the recent update for the camera app waiting.
I updated mine, will have to see if it stops happening.
They also say on here you can turn "put app to sleep" off when not in use or something in the apps' settings, but I want the app to be put to sleep.
And when it happens, you can long press the camera app icon and select "force stop" to resolve it at the moment.