Crash when a militarysite window is open while the enemy conquers it

When a militarysite windows is open and the enemy conquers it, widelands crashes.

#0 0x0000000000000000 in ?? ()
#1 0x0000000001106caa in BuildingWindow::think (this=0xaadeff0)
    at trunk/src/wui/buildingwindow.cc:147
#2 0x0000000000febadb in UI::Panel::do_think (this=0xaadeff0)
    at trunk/src/ui_basic/panel.cc:458
#3 0x0000000000febafa in UI::Panel::do_think (this=0x3a97f40)
    at trunk/src/ui_basic/panel.cc:461
#4 0x0000000000feb0f7 in UI::Panel::do_run (this=0x3a97f40)
    at trunk/src/ui_basic/panel.cc:183
#5 0x0000000000d12e5e in UI::Panel::run<UI::Panel::Returncodes> (
    this=0x3a97f40)
    at trunk/src/ui_basic/panel.h:99
#6 0x0000000000e3d934 in Widelands::Game::run (this=0x7fffffffa160, loader_ui=
    0x7fffffff9ff0, start_game_type=Widelands::Game::NewNonScenario,
    script_to_run="", replay=false, prefix_for_replays="single_player")
    at trunk/src/logic/game.cc:524
#7 0x0000000000d0cdf5 in WLApplication::new_game (this=0x1a55d50)
    at trunk/src/wlapplication.cc:1248
#8 0x0000000000d0c22f in WLApplication::mainmenu_singleplayer (this=0x1a55d50)
    at trunk/src/wlapplication.cc:1103
#9 0x0000000000d0bb61 in WLApplication::mainmenu (this=0x1a55d50)
    at trunk/src/wlapplication.cc:1008
#10 0x0000000000d08692 in WLApplication::run (this=0x1a55d50)
   /sources/widelands/trunk/src/wlapplication.cc:445
#11 0x0000000000d06ab1 in main (argc=1, argv=0x7fffffffdd58) at trunk/src/main.cc:49