BuddyPress.org

You can get rid of the error by calling that like: bp_core_widget_whos_online(array()); but it doesn’t pay any attention to args being passed when it comes to number of user to display. You’ll have to fire up the widget for each blog, set the max users, deactivate the widget to not display it. This widget was not designed to be used in this way.

You’d be better off creating a mini template for this purpose.

https://codex.buddypress.org/developer-docs/custom-buddypress-loops/the-site-members-loop/ with ‘type=online’.