With the version currently in cvs:

- start up
- click "restbreak"
Pooof

>>> RestBreakWindow::RestBreakWindow
<<< RestBreakWindow::RestBreakWindow
>>> RestBreakWindow::start
>>> System::init
    System::init Locking disabled
<<< System::init
 
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 16384 (LWP 15614)]
0x08099d26 in Gtk::Button* SigC::manage<Gtk::Button>(Gtk::Button*) (t=0x0)
    at object.h:116
116         { t->set_manage(); return t; }
(gdb) where
#0  0x08099d26 in Gtk::Button* SigC::manage<Gtk::Button>(Gtk::Button*) (t=0x0)
    at object.h:116
#1  0x080997b9 in BreakWindow::create_break_buttons(bool, bool) (
    this=0x8269b08, lockable=true, shutdownable=false) at BreakWindow.cc:442
#2  0x080b0efe in RestBreakWindow::create_gui() (this=0x8269b08)
    at RestBreakWindow.cc:93
#3  0x08098475 in BreakWindow::init_gui() (this=0x8269b08) at BreakWindow.cc:88
#4  0x080b1421 in RestBreakWindow::start() (this=0x8269b08)
    at RestBreakWindow.cc:116
#5  0x0809d9bc in GUI::start_break_window(BreakId, bool) (this=0x8167880,
    break_id=BREAK_ID_REST_BREAK, ignorable=true) at GUI.cc:846
#6  0x08104692 in BreakControl::break_window_start() (this=0x81c2d18)
    at BreakControl.cc:648
#7  0x08103954 in BreakControl::goto_stage(BreakControl::BreakStage) (
    this=0x81c2d18, stage=STAGE_TAKING) at BreakControl.cc:298
#8  0x08104018 in BreakControl::force_start_break() (this=0x81c2d18)
    at BreakControl.cc:445
#9  0x080e0899 in Core::break_action(BreakId, Core::BreakAction) (
    this=0x81bd280, id=BREAK_ID_REST_BREAK,
    action=BREAK_ACTION_FORCE_START_BREAK) at Core.cc:849
#10 0x080df850 in Core::force_break(BreakId) (this=0x81bd280,
    id=BREAK_ID_REST_BREAK) at Core.cc:455
#11 0x0809b982 in GUI::restbreak_now() (this=0x8167880) at GUI.cc:173
#12 0x080caef2 in Menus::on_menu_restbreak_now() (this=0x81fdba8)
    at Menus.cc:395
#13 0x080c508f in SigC::ObjectSlot0_<void, Menus>::proxy(void*) (s=0x8223350)
    at object_slot.h:62
#14 0x40c7ac35 in Glib::SignalProxyNormal::slot0_void_callback(_GObject*, void*)
() from /usr/lib/libglibmm-2.0.so.1
#15 0x40750212 in g_cclosure_marshal_VOID__VOID ()
   from /usr/lib/libgobject-2.0.so.0
#16 0x4073fa92 in g_closure_invoke () from /usr/lib/libgobject-2.0.so.0
#17 0x4074fc6d in g_signal_emit_by_name () from /usr/lib/libgobject-2.0.so.0
#18 0x4074eec2 in g_signal_emit_valist () from /usr/lib/libgobject-2.0.so.0
#19 0x4074f0d6 in g_signal_emit () from /usr/lib/libgobject-2.0.so.0
#20 0x40337537 in gtk_button_clicked () from /usr/lib/libgtk-x11-2.0.so.0
#21 0x403384fb in _gtk_button_paint () from /usr/lib/libgtk-x11-2.0.so.0
#22 0x40a751f9 in Gtk::Button_Class::released_callback(_GtkButton*) ()
   from /usr/lib/libgtkmm-2.0.so.1
#23 0x40750212 in g_cclosure_marshal_VOID__VOID ()
---Type <return> to continue, or q <return> to quit---
   from /usr/lib/libgobject-2.0.so.0
#24 0x4073fd44 in g_cclosure_new_swap () from /usr/lib/libgobject-2.0.so.0
#25 0x4073fa92 in g_closure_invoke () from /usr/lib/libgobject-2.0.so.0
#26 0x4074f6c4 in g_signal_emit_by_name () from /usr/lib/libgobject-2.0.so.0
#27 0x4074eec2 in g_signal_emit_valist () from /usr/lib/libgobject-2.0.so.0
#28 0x4074f0d6 in g_signal_emit () from /usr/lib/libgobject-2.0.so.0
#29 0x40337487 in gtk_button_released () from /usr/lib/libgtk-x11-2.0.so.0
#30 0x4033837b in _gtk_button_paint () from /usr/lib/libgtk-x11-2.0.so.0
#31 0x40b02ac2 in Gtk::Widget_Class::button_release_event_callback(_GtkWidget*,
_GdkEventButton*) () from /usr/lib/libgtkmm-2.0.so.1
#32 0x403bf282 in _gtk_marshal_BOOLEAN__BOXED ()
   from /usr/lib/libgtk-x11-2.0.so.0
#33 0x4073fd44 in g_cclosure_new_swap () from /usr/lib/libgobject-2.0.so.0
#34 0x4073fa92 in g_closure_invoke () from /usr/lib/libgobject-2.0.so.0
#35 0x4074f895 in g_signal_emit_by_name () from /usr/lib/libgobject-2.0.so.0
#36 0x4074eca3 in g_signal_emit_valist () from /usr/lib/libgobject-2.0.so.0
#37 0x4074f0d6 in g_signal_emit () from /usr/lib/libgobject-2.0.so.0
#38 0x4049dc6b in gtk_widget_send_expose () from /usr/lib/libgtk-x11-2.0.so.0
#39 0x403bf057 in gtk_propagate_event () from /usr/lib/libgtk-x11-2.0.so.0
#40 0x403bddb6 in gtk_main_do_event () from /usr/lib/libgtk-x11-2.0.so.0
#41 0x40577f85 in _gdk_events_queue () from /usr/lib/libgdk-x11-2.0.so.0
#42 0x4078c086 in unblock_source () from /usr/lib/libglib-2.0.so.0
#43 0x4078d029 in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#44 0x4078d347 in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#45 0x4078d9b0 in g_main_loop_run () from /usr/lib/libglib-2.0.so.0
#46 0x403bd5df in gtk_main () from /usr/lib/libgtk-x11-2.0.so.0
#47 0x40aa49a9 in Gtk::Main::run_impl() () from /usr/lib/libgtkmm-2.0.so.1
#48 0x40aa4744 in Gtk::Main::run() () from /usr/lib/libgtkmm-2.0.so.1
#49 0x0809ba91 in GUI::main() (this=0x8167880) at GUI.cc:215
#50 0x080cd0bf in run (argc=1, argv=0xbffffd04) at main.cc:44
#51 0x080cd10f in main (argc=1, argv=0xbffffd04) at main.cc:83
(gdb)

Fixed.