[ash-md] Prevents a crash when entering overview mode while dragging a window

ash/common/wm/overview/window_selector_item.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/common/wm/overview/window_selector_item.h"
 
 #include <algorithm>
 #include <vector>
 
 #include "ash/common/material_design/material_design_controller.h"
@@ skipping 409 matching lines @@
 }
 
 void WindowSelectorItem::Shutdown() {
   if (transform_window_.GetTopInset()) {
     // Activating a window (even when it is the window that was active before
     // overview) results in stacking it at the top. Maintain the label window
     // stacking position above the item to make the header transformation more
     // gradual upon exiting the overview mode.
     WmWindow* label_window =
         WmLookup::Get()->GetWindowForWidget(window_label_.get());
-    label_window->GetParent()->StackChildAbove(label_window,
-                                               transform_window_.window());
+
+    // |label_window| was originally created in the same container as the
+    // |transform_window_| but when closing overview the |transform_window_|
+    // could have been reparented if a drag was active. Only change stacking
+    // if the windows still belong to the same container.
+    if (label_window->GetParent() == transform_window_.window()->GetParent()) {
+      label_window->GetParent()->StackChildAbove(label_window,
+                                                 transform_window_.window());
+    }
   }
   if (background_view_) {
     background_view_->OnItemRestored();
     background_view_ = nullptr;
   }
   FadeOut(std::move(window_label_));
 }
 
 void WindowSelectorItem::ShowWindowOnExit() {
   transform_window_.ShowWindowOnExit();
@@ skipping 339 matching lines @@
   window_selector_->delegate()->AddDelayedAnimationObserver(
       std::move(observer));
   widget_ptr->SetOpacity(0.f);
 }
 
 gfx::SlideAnimation* WindowSelectorItem::GetBackgroundViewAnimation() {
   return background_view_ ? background_view_->animation() : nullptr;
 }
 
 }  // namespace ash