You aren't supposed to get this error, if this is something you are seeing consistently you should definitely look in to this further.
To give you some context, this is likely an action that is triggered in parallel with the logout request. It could also be something as simple as a microflow timer or datagrid refresh action which triggers at the exact same time the user logs in.
So the session is still valid when the browser sends the action to start the microflow or retrieve the data, but before the server receive the request the logout has been processed and the session has been removed.
If this is an one time incident you could ignore it, but if you see this more often I would recommend to look into this because error are never a good thing.