Here is info from dump:

Core was generated by `xfce4-panel --sm-client-id 238a0f92e-43e5-4ffd-bda7-5af9ac476a0c'.
Program terminated with signal 11, Segmentation fault.
#0  0x00007fd5e4e36cbc in xfce_tasklist_button_enter_notify_event_disconnected (data=0xbe0340, closure=0xa8a640) at tasklist-widget.c:2664
2664	  panel_return_if_fail (WNCK_IS_WINDOW (child->window));
(gdb) bt full
#0  0x00007fd5e4e36cbc in xfce_tasklist_button_enter_notify_event_disconnected (data=0xbe0340, closure=0xa8a640) at tasklist-widget.c:2664
        __inst = 0x100000001
        __t = 7484144
        __r = 32725
        child = 0xbe0340
        __PRETTY_FUNCTION__ = "xfce_tasklist_button_enter_notify_event_disconnected"
#1  0x00007fd5ef2ebf40 in g_closure_unref () from /usr/lib64/libgobject-2.0.so.0
No symbol table info available.
#2  0x00007fd5ef30668d in g_signal_handlers_destroy () from /usr/lib64/libgobject-2.0.so.0
...
#16 0x00007fd5ef2f0fa0 in g_object_run_dispose () from /usr/lib64/libgobject-2.0.so.0
No symbol table info available.
#17 0x00007fd5e4e31033 in xfce_tasklist_arrow_button_menu_destroy (menu=0xb22550, tasklist=0x71e2b0) at tasklist-widget.c:1302
        __PRETTY_FUNCTION__ = "xfce_tasklist_arrow_button_menu_destroy"
#18 0x00007fd5ef2ec795 in g_closure_invoke () from /usr/lib64/libgobject-2.0.so.0
No symbol table info available.
#19 0x00007fd5ef2feeb1 in ?? () from /usr/lib64/libgobject-2.0.so.0
No symbol table info available.
#20 0x00007fd5ef3081a4 in g_signal_emit_valist () from /usr/lib64/libgobject-2.0.so.0
No symbol table info available.
...
#35 0x00007fd5f0d5ea67 in gtk_main () from /usr/lib64/libgtk-x11-2.0.so.0
No symbol table info available.
#36 0x0000000000411bc0 in main (argc=1, argv=0x7fff4f256668) at main.c:360
        context = 0x67c500
        application = 0x6a1e30
        error = 0x0
        dbus_service = 0x6a26a0
        succeed = 0
        i = 5
        signums = {2, 3, 15, 6, 10}
        error_msg = 0x0
        sm_client = 0x6b80c0
(gdb) f 0
#0  0x00007fd5e4e36cbc in xfce_tasklist_button_enter_notify_event_disconnected (data=0xbe0340, closure=0xa8a640) at tasklist-widget.c:2664
2664	  panel_return_if_fail (WNCK_IS_WINDOW (child->window));
(gdb) p child
$6 = (XfceTasklistChild *) 0xbe0340
(gdb) p *child
$7 = {type = 9633552, tasklist = 0x0, button = 0xc0a190, box = 0xbeb9a0, icon = 0xbeba40, label = 0x0, motion_timeout_id = 12785024, motion_timestamp = 0, 
  unique_id = 12785024, last_focused = {tv_sec = 263882790666240, tv_usec = 1}, windows = 0x2800, window = 0x100000001, class_group = 0x17c0000000000}
(gdb) p child->window
$8 = (WnckWindow *) 0x100000001
(gdb) p *(child->window)
Cannot access memory at address 0x100000001
(gdb) p child->windows
$9 = (GSList *) 0x2800
(gdb) p *(child->windows)
Cannot access memory at address 0x2800