How to reproduce : open the "add items" dialog, select "Task list", then 
"Add".

Result : segfault

Workaround : drag&drop the Tasklist plugin from the "add items" list to 
a place in the panel : result OK

Reproducible: Always
Steps to Reproduce:
1. open the "add items" dialog
2. select "Task list"
3. Click "Add"
Actual Results:  
segfault

Expected Results:  
Tasklist is added to the panel

Here comes the backtrace :

(xfce4-panel:1694): GLib-GObject-CRITICAL **: g_object_freeze_notify: 
assertion `G_IS_OBJECT (object)' failed

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread -1217168736 (LWP 1694)]
0xb7dc71e1 in gtk_label_get_mnemonic_keyval () from 
/usr/lib/libgtk-x11-2.0.so.0
(gdb) bt
#0  0xb7dc71e1 in gtk_label_get_mnemonic_keyval ()
   from /usr/lib/libgtk-x11-2.0.so.0
#1  0xb7dc9bf6 in gtk_label_set_text () from /usr/lib/libgtk-x11-2.0.so.0
#2  0xb7f91bb0 in netk_tasklist_get_size_hint_list ()
   from /usr/lib/libxfcegui4.so.4
#3  0xb79312f4 in g_cclosure_marshal_VOID__VOID ()
   from /usr/lib/libgobject-2.0.so.0
#4  0xb7919d5c in g_closure_invoke () from /usr/lib/libgobject-2.0.so.0
#5  0xb792fadc in g_signal_has_handler_pending () from 
/usr/lib/libgobject-2.0.so.0
#6  0xb79309ef in g_signal_emit_valist () from /usr/lib/libgobject-2.0.so.0
#7  0xb7930d5b in g_signal_emit () from /usr/lib/libgobject-2.0.so.0
#8  0xb7f981bb in p_netk_window_process_configure_notify ()
   from /usr/lib/libxfcegui4.so.4
#9  0xb7f98cd7 in p_netk_window_process_configure_notify ()
   from /usr/lib/libxfcegui4.so.4
#10 0xb7f990c5 in p_netk_window_process_configure_notify ()
   from /usr/lib/libxfcegui4.so.4
#11 0xb78ad7c1 in g_child_watch_add () from /usr/lib/libglib-2.0.so.0
#12 0xb78aa524 in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#13 0xb78abf56 in g_main_context_acquire () from /usr/lib/libglib-2.0.so.0
#14 0xb78ac1d5 in g_main_loop_run () from /usr/lib/libglib-2.0.so.0
---Type <return> to continue, or q <return> to quit---
#15 0xb7dd57c3 in gtk_main () from /usr/lib/libgtk-x11-2.0.so.0
#16 0x0804ffa4 in panel_app_run ()
#17 0x0804f412 in main ()