That part of the manual is way out of date, I'll fix it. The problem here is for other developers who use WP_DEBUG to catch coding mistakes. You have minor warnings on other Dashboard pages for undefined variables indexes and stuff, but this particular one pollutes all Dashboard pages, so really gets in the way. I'm on the verge of writing my own script to fix your code each time you push an update.
Steps to reproduce (plugin ver. 7.0.4):
define('WP_DEBUG', true) in wp-config.php.
2. Dashboard / Statistics / Settings / Show stats in menu bar / true
Notice: WP_Admin_Bar::add_node was called incorrectly. The menu ID should not be empty. Please see Debugging in WordPress for more information. (This message was added in version 3.3.) in /../wp-includes/functions.php on line 3245
(Repeated 6 times.)
Also, there's a ton of files in wp-content/plugins/wp-statistcs/vendor/ you can leave out. GeoIP is great, a geo lookup that doesn't need giant db tables, and it's just as fast in my testing (can't recall which was faster, they were close enough). But GeoIP distributes with that composer junk: guzzle, symfony and all that. Probably none of it is used. I figured out how to get rid of it in my plugins, could probably do the same in yours. Tell me if you want the code.