From time to time on my x86 (for amd64 I don't remember such error) workstation I see the issue with x11-terms/terminal. Application completely looses GUI (so, could be terminated only from real console). XSession log (/var/log/slim.log) at this time conmtains teh following record. (Terminal:2991): Gtk-CRITICAL **: IA__gtk_widget_event: assertion `WIDGET_REALIZED_FOR_EVENT (widget, event)' failed The issue appears rather often (in my case from several times a month up to once at several mobths), but is not willingly reproducable (so I'm not shure, that could provide backtrace).
Without a backtrace, not possible to fix. Reopen if you have one.