Steps to reproduce:
Try to create a task, evolution crashes with the message:

Gtk-CRITICAL **: file gtkwidget.c: line 4425: assertion `GTK_IS_WIDGET (widget)'
failed
aborting...

The root cause is two useless widgets: sendoptions_frame and sendoptions_button.


Stack trace:
 [1] _waitid(0x0, 0x4e25, 0x8046220, 0x3), at 0xce709a25
  [2] _waitpid(0x4e25, 0x8046334, 0x0), at 0xce6d577f
  [3] waitpid(0x4e25, 0x8046334, 0x0), at 0xce6fd107
  [4] libgnomeui_segv_handle(0x6, 0x0, 0x80463e0), at 0xd0380aee
  [5] __sighndlr(0x6, 0x0, 0x80463e0, 0xd0380a44), at 0xce7080df
  [6] __lwp_kill(0x1, 0x6), at 0xce7095f5
  [7] _thr_kill(0x1, 0x6), at 0xce705fc9
  [8] raise(0x6), at 0xce6cdfdd
  [9] abort(0x843ae90, 0xcfdde580, 0xce7f0eb0, 0x8001, 0x8092168, 0x0), at
0xce6b036f
  [10] g_logv(0xcfdde580, 0x8, 0xcfde00c8, 0x8046b1c), at 0xce7923e4
  [11] g_log(0xcfdde580, 0x8, 0xcfde00c8, 0xcfddf238, 0x1149, 0xcfddf220), at
0xce792411
  [12] gtk_widget_set_sensitive(0x0, 0x0), at 0xcfd3650e
=>[13] sensitize_widgets(tpage = 0x8379700), line 404 in "task-page.c"
  [14] task_page_set_assignment(page = 0x8379700, set = 0), line 474 in
"task-page.c"
  [15] task_editor_construct(te = 0x843ae90, client = 0x83c93c0), line 456 in
"task-editor.c"
  [16] task_editor_new(client = 0x83c93c0, flags = COMP_EDITOR_NEW_ITEM), line
654 in "task-editor.c "
  [17] gnome_calendar_new_task(gcal = 0x8323c68), line 2961 in "gnome-cal.c"
  [18] on_new_task(ep = 0x8336728, pitem = 0xcc58da70, data = 0x839e908), line
1053 in "e-calendar-v iew.c"
  [19] ep_activate(w = 0x85ba9c0, inode = 0x852fba8), line 304 in "e-popup.c"
  [20] g_cclosure_marshal_VOID__VOID(0x84360e0, 0x0, 0x1, 0x8046e5c, 0x8046dbc,
0x0), at 0xcf69e321
  [21] g_closure_invoke(0x84360e0, 0x0, 0x1, 0x8046e5c, 0x8046dbc), at 0xcf68a256
  [22] signal_emit_unlocked_R(0x81dd670, 0x0, 0x85ba9c0, 0x0, 0x8046e5c), at
0xcf69dba0
  [23] g_signal_emit_valist(0x85ba9c0, 0xa8, 0x0, 0x80470c8), at 0xcf69cea5
  [24] g_signal_emit(0x85ba9c0, 0xa8, 0x0), at 0xcf69d041
  [25] gtk_widget_activate(0x85ba9c0), at 0xcfd3566c
  [26] gtk_menu_shell_activate_item(0x83c9128, 0x85ba9c0, 0x1), at 0xcfc61dfa
  [27] gtk_menu_shell_button_release(0x83c9128, 0x813e7f4), at 0xcfc61271
  [28] gtk_menu_button_release(0x83c9128, 0x813e7f4, 0x80a0530), at 0xcfc5a417
  [29] _gtk_marshal_BOOLEAN__BOXED(0x81cec40, 0x8047270, 0x2, 0x804732c,
0x804728c, 0xcfc5a3c0), at 0xcfc53c9d
  [30] g_type_class_meta_marshal(0x81cec40, 0x8047270, 0x2, 0x804732c,
0x804728c, 0xb4), at 0xcf68a5 51
  [31] g_closure_invoke(0x81cec40, 0x8047270, 0x2, 0x804732c, 0x804728c), at
0xcf68a256
  [32] signal_emit_unlocked_R(0x81cefd0, 0x0, 0x83c9128, 0x80474ac, 0x804732c),
at 0xcf69dd65
  [33] g_signal_emit_valist(0x83c9128, 0x54, 0x0, 0x80475a0), at 0xcf69cc3b
  [34] g_signal_emit(0x83c9128, 0x54, 0x0, 0x813e7f4, 0x80475c4), at 0xcf69d041
  [35] gtk_widget_event_internal(0x83c9128, 0x813e7f4), at 0xcfd354de
  [36] gtk_widget_event(0x83c9128, 0x813e7f4), at 0xcfd3516d
  [37] gtk_propagate_event(0x83c9128, 0x813e7f4), at 0xcfc52812
  [38] gtk_main_do_event(0x813e7f4, 0x0), at 0xcfc51884
  [39] gdk_event_dispatch(0x80b0380, 0x0, 0x0), at 0xcf8ce03a
  [40] g_main_dispatch(0x80b03c8), at 0xce789ca4
  [41] g_main_context_dispatch(0x80b03c8), at 0xce78ad95
  [42] g_main_context_iterate(0x80b03c8, 0x1, 0x1, 0x80928a8), at 0xce78b1b5
  [43] g_main_loop_run(0x81a0278), at 0xce78b7ba
  [44] bonobo_main(0x80479bc, 0x80478e4, 0xd0ffb840, 0x8047830, 0x80478a0,
0x809936c), at 0xd00bf462 
  [45] main(argc = 1, argv = 0x8047928), line 610 in "main.c"


Other information:
I will provide a patch soon