To Reproduce:

1) Attempt to remove Window List/WorkSpaceSwitcher from panel, by selecting
"Remove from Panel" popup option.

Panel crashes, stack trace below:

$ pstack 2251
2251:   gnome-panel-2
-----------------  lwp# 1 / thread# 1  --------------------
 fdd9c2c8 waitid   (0, 903, ffbeeaf8, 3)
 fdd563a8 _waitpid (0, ffbeec00, 0, ffbeec00, 0, 0) + 60
 feadaab0 waitpid  (a, ffbef058, ffbeeda0, 0, 0, 0) + 34
 feadb7dc __sighndlr (a, ffbef058, ffbeeda0, ff18d800, 12f284, 12f274) + c
 fead84c0 sigacthandler (a, 12f1e0, 0, 0, 0, feaee000) + 708
 --- called from signal handler with signal 10 (SIGBUS) ---
 ff3538e0 wnck_tasklist_active_window_changed (1543d8, 1f38e8, 1, 1, 0, 0)
+ 48
 fdf49e50 g_cclosure_marshal_VOID__VOID (1fe5f0, 0, 1f38e8, 1543d8, 1fe5f0,
ff353898) + 150
 fdf23220 g_closure_invoke (1fe5f0, 0, 1, ffbef448, ffbef30c, 0) + 278
 fdf4924c signal_emit_unlocked_R (155cb8, 0, 1543d8, 0, ffbef448, 0) + c6c
 fdf47d2c g_signal_emit_valist (1543d8, f, 0, ffbef788, 142070, 0) + b44
 fdf481a4 g_signal_emit (1543d8, f, 0, 4, 4, 0) + 64
 ff3502bc emit_active_window_changed (1543d8, ffbef790, ffbef78c, fddb801c,
0, 0) + 54
 ff34fe2c update_active_window (1543d8, 0, 0, fde6b200, 0, 0) + 104
 ff35009c do_update_now (1543d8, c332, 124cb6, 0, 0, ffbef8d8) + 8c
 ff350114 update_idle (1543d8, 14cb18, 1, fde5d44c, 0, 0) + 44
 fde65c84 g_idle_dispatch (2a0330, ff3500d0, 1543d8, fde60234, 0, 0) + 84
 fde60260 g_main_dispatch (14cb18, 0, 0, fde620f8, 0, 0) + 270
 fde62218 g_main_context_dispatch (14cb18, c8, 1fd010, 7, 7, 0) + c8
 fde62978 g_main_context_iterate (14cb18, 1, 1, 12fa98, ff3e2660, 3b867) +
6b8
 fde639a0 g_main_loop_run (1d1b68, 1d1b68, 40a, 0, 0, 0) + 5c0
 fe1996b0 gtk_main (20, 4f4e0, 0, 0, 0, 0) + 1c0
 0004ff18 main     (f50a4, 120904, 120914, 120924, 120938, 120944) + 2c8
 0004c7d8 _start   (0, 0, 0, 0, 0, 0) + 108
-----------------  lwp# 2 / thread# 2  --------------------
 fdd9b684 signotifywait ()
 feaced4c _dynamiclwps (feaee000, 59, 0, 0, ffbef5fc, 4) + 1c
 fead2028 thr_yield (0, 0, 0, 0, 0, 0) + 8c
-----------------  lwp# 3  --------------------------------
 fdd99228 door     (0, 0, 0, 0, fdc45d10, 4)
 fead6b9c _sc_door_func (0, 0, 0, 0, 0, 0) + 54
-----------------  lwp# 4  --------------------------------
 fdd9bce8 lwp_cond_wait (feaf5550, feaf5560, feaeedb8)
 feac9098 _age     (4, feaeeda4, feaee000, 3, feaee000, 1) + 74
 feaca72c _lwp_start (fdc33d70, 0, 4000, fdc0fc2c, 0, 0) + 18
 fead2028 thr_yield (0, 0, 0, 0, 0, 0) + 8c
--------------------------  thread# 3  --------------------
 feacddb4 _reap_wait (feaf29e8, 20528, 0, feaee000, 0, 0) + 38
 feacdb0c _reaper  (feaeee38, feaf4748, feaf29e8, feaeee10, 1, fe400000) +
38
 feadb6e0 _thread_start (0, 0, 0, 0, 0, 0) + 40
$