Apologies for not writing in German, but for future reference: on Ubuntu 18.04.1, soon after the upgrade from 16.04 (but not immediately, I'd played with tray widgets), I had the exact same symptom
Jul 31 21:49:17 marnux2 org.gnome.Shell.desktop[13830]: Gtk:ERROR:../../../../gtk/gtkicontheme.c:4261:gtk_icon_info_load_icon_finish: assertion failed: (icon_info_get_pixbuf_ready (icon_info))
+ crash of that gnome-shell process, and it was resolved after only uninstalling package indicator-multiload.
I have no idea what went wrong or why this helped, but I hope this helps someone...