Fix crash when removing all notifications by "Clear all" button.

ui/arc/notification/arc_custom_notification_view.cc

 // Copyright 2016 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 "ui/arc/notification/arc_custom_notification_view.h"
 
 #include "ash/wm/window_util.h"
 #include "base/auto_reset.h"
 #include "base/memory/ptr_util.h"
 #include "components/exo/notification_surface.h"
@@ skipping 151 matching lines @@
     return owner_->close_button_->HasFocus();
   }
 
   void RequestFocusOnCloseButton() override {
     if (owner_->close_button_)
       owner_->close_button_->RequestFocus();
     owner_->UpdateControlButtonsVisibility();
   }
 
   bool IsPinned() const override {
+    if (!owner_->item_)
+      return false;
     return owner_->item_->pinned();
   }
 
   void UpdateControlButtonsVisibility() override {
     owner_->UpdateControlButtonsVisibility();
   }
 
  private:
   ArcCustomNotificationView* const owner_;
 
@@ skipping 405 matching lines @@
 
 void ArcCustomNotificationView::OnNotificationSurfaceRemoved(
     exo::NotificationSurface* surface) {
   if (surface->notification_id() != notification_key_)
     return;
 
   SetSurface(nullptr);
 }
 
 }  // namespace arc