Easy reproducer: start `geany -c nonexistant` do `File->New` then quit.
The backtrace looks like the above each time, and there is no Geany or Scintilla code in there (except `do_main_quit` and `main_quit` which is expected). Its happening on `gtk_widget_destroy()` of the main window so maybe Scintilla has changed some object lifetime and GTK is trying to release a dangling pointer.