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..1aae81dd6bfde73471ed56e085012cedba6075d4 100644
--- a/ash/root_window_controller.cc
+++ b/ash/root_window_controller.cc
@@ -464,15 +464,19 @@ void RootWindowController::ShowContextMenu(
   if (!menu_model.get())
     return;
 
-  views::MenuRunner menu_runner(menu_model.get());
-  views::Widget* widget =
-      root_window_->GetProperty(kDesktopController)->widget();
-
-  if (menu_runner.RunMenuAt(
-          widget, NULL, gfx::Rect(location_in_screen, gfx::Size()),
-          views::MenuItemView::TOPLEFT, views::MenuRunner::CONTEXT_MENU) ==
-      views::MenuRunner::MENU_DELETED)
+  internal::DesktopBackgroundWidgetController* background =
+      root_window_->GetProperty(kDesktopController);
+  views::Widget* widget = background ? background->widget() : NULL;

[oshima, 2013/04/03 15:14:12]

when it can become null?

[Dmitry Polukhin, 2013/04/03 18:50:00]

I wasn't able to reproduce this crash but you can see stack trace in the bug
https://code.google.com/p/chromium/issues/detail?id=222218

The crash happens because background controller is NULL. I see several places
where it can be set to NULL (on place in this file in
RootWindowController::CloseChildWindows). It looks like it is set to NULL when
windows are destroying. But I suspect the windows still could receive message. I
think we can remove check for widget but I added it to be on safe side.

+  if (widget) {
+    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) ==
+        views::MenuRunner::MENU_DELETED)
+      return;
+  } else {
     return;
+  }

[oshima, 2013/04/03 15:14:12]

if (!widget)
  return;
if (menu_runner.RunMenuAt(....)) {
  return;
}

is easier to follow

[Dmitry Polukhin, 2013/04/03 18:50:00]

Done.

 
   Shell::GetInstance()->UpdateShelfVisibility();
 }