Gajim git master 06.06.2018
From this commit 970d6f8c it is hard to say, why it happened. It is too big.
prepare_context_menu() from here 970d6f8c has been removed. This must have been resulted in an exception when invoking the context menu, which has been just silenced here 970d6f8c.
The UI file is still there: https://dev.gajim.org/gajim/gajim/blob/3c1490a3066515ff1f9adaff7f9dc102f0ace341/gajim/data/gui/gc_control_popup_menu.ui