This error leads to no application icon and no notification icon which is kind of annoying. It seems that the cause is that gdk-pixbuf-2.36.1 removed pixdata. See this arch post speaking about this: https://bbs.archlinux.org/viewtopic.php?id=220697 And the upstream: https://bugzilla.gnome.org/show_bug.cgi?id=776004#c18
And this upstream bug filled by hexchat dev: https://bugzilla.gnome.org/show_bug.cgi?id=776105
fixed in =x11-libs/gdk-pixbuf-2.36.2