The systemMonitor extension extends the message tray with its UI, which
obviously broke when the bottom tray was removed. It has also been the
source for various performance/memory issues in the past and is part of
the semi-random collection that pre-dates the extension.gnome.org site
rather than the set of classic-mode/endorsed extensions, so just remove
it from the module.
Hopefully this will motivate someone who actually cares about it to pick
it up ...

https://bugzilla.gnome.org/show_bug.cgi?id=745195