Appmenu: clicking WM close button causes bridge.c:718::child_notify_cb() to exit

Binary package hint: appmenu-gtk

Sometimes hitting the WM Close "(X)" button on a single component window causes the whole application to exit. Easiest way to reproduce is to load several images in GIMP at the same time, and then start to close them one-by-one. 50% of the time this causes GIMP to exit via appmenu-gtk/child_notify_cb():

#0 g_type_check_instance_is_a (type_instance=0x91e0800, iface_type=154036392) at /build/buildd/glib2.0-2.27.4/gobject/gtype.c:3941
#1 0x0155d3e7 in child_notify_cb (widget=0x8d6ca78, pspec=0x84b8000, mi=0x934e6f0) at bridge.c:718
#2 0x00b6f38c in g_cclosure_marshal_VOID__PARAM (closure=0x9354858, return_value=0x0, n_param_values=2, param_values=0x91e0800,
    invocation_hint=0xbfffd770, marshal_data=0x0) at /build/buildd/glib2.0-2.27.4/gobject/gmarshal.c:533

(gdb) print ...
*widget = {object = {parent_instance = {g_type_instance = {g_class = 0x8ad2a00}, ref_count = 21, qdata = 0x8d67710}, flags = 2164512}, private_flags = 15360, state = 0 '\000', saved_state = 0 '\000', name = 0x8d74a20 "Tools", style = 0x8f09408, requisition = { width = 0, height = 0}, allocation = {x = 1, y = 1, width = 1, height = 1}, window = 0x0, parent = 0x0}
*pspec = {g_type_instance = {g_class = 0x84b7dd0}, name = 0x68bb27 "parent", flags = 227, value_type = 139227816, owner_type = 139288064, _nick = 0x6822d4 "Parent widget", _blurb = 0x6831ec "The parent widget of this widget. Must be a Container widget", qdata = 0x0, ref_count = 3, param_id = 2}
*mi = {parent = {g_type_instance = {g_class = 0x92f1088}, ref_count = 1, qdata = 0x9353340}}

Higher-up the stack are calls to gtk_widget_unparent() which presumably sets off the sequence.