Prevent crash when desktop controller is NULL

ash/root_window_controller.cc

Index: ash/root_window_controller.cc
diff --git a/ash/root_window_controller.cc b/ash/root_window_controller.cc
index 3af8f489002c05f1b530354e6ab9b435037b7505..7a6271a1c8dc98f1e6184a47dd2aa231cf339859 100644
--- a/ash/root_window_controller.cc
+++ b/ash/root_window_controller.cc
@@ -464,10 +464,16 @@ void RootWindowController::ShowContextMenu(
   if (!menu_model.get())
     return;
 
-  views::MenuRunner menu_runner(menu_model.get());
-  views::Widget* widget =
-      root_window_->GetProperty(kDesktopController)->widget();
+  internal::DesktopBackgroundWidgetController* background =
+      root_window_->GetProperty(kDesktopController);
+  if (!background)

[oshima, 2013/04/04 13:44:51]

can you explain when this happens in the comment with a reference to the bug?

[Dmitry Polukhin, 2013/04/04 14:42:45]

Done.

+    return;
 
+  views::Widget* widget = background->widget();
+  if (!widget)
+    return;

[oshima, 2013/04/04 13:44:51]

can this happen?

[Dmitry Polukhin, 2013/04/04 14:42:45]

I haven't seen this - removed.

+
+  views::MenuRunner menu_runner(menu_model.get());
   if (menu_runner.RunMenuAt(
           widget, NULL, gfx::Rect(location_in_screen, gfx::Size()),
           views::MenuItemView::TOPLEFT, views::MenuRunner::CONTEXT_MENU) ==