Fix occasional crash in ~VPNListView during shell destruction.

ash/system/chromeos/network/vpn_list_view.cc

 // Copyright 2015 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/system/chromeos/network/vpn_list_view.h"
 
 #include <utility>
 #include <vector>
 
 #include "ash/metrics/user_metrics_recorder.h"
@@ skipping 221 matching lines @@
 
 }  // namespace
 
 VPNListView::VPNListView(ui::NetworkListDelegate* delegate)
     : delegate_(delegate) {
   Shell::GetInstance()->system_tray_delegate()->GetVPNDelegate()->AddObserver(
       this);
 }
 
 VPNListView::~VPNListView() {
-  Shell::GetInstance()
-      ->system_tray_delegate()
-      ->GetVPNDelegate()
-      ->RemoveObserver(this);
+  // We need the check as on shell destruction, the delegate is destroyed first.
+  SystemTrayDelegate* const system_tray_delegate =
+      Shell::GetInstance()->system_tray_delegate();
+  if (system_tray_delegate) {
+    VPNDelegate* const vpn_delegate = system_tray_delegate->GetVPNDelegate();
+    if (vpn_delegate)
+      vpn_delegate->RemoveObserver(this);
+  }
 }
 
 void VPNListView::Update() {
   // Before updating the list, determine whether the user was hovering over one
   // of the VPN provider or network entries.
   scoped_ptr<VPNProvider::Key> hovered_provider_key;
   std::string hovered_network_service_path;
   for (const std::pair<const views::View* const, VPNProvider::Key>& provider :
        provider_view_key_map_) {
     if (static_cast<const HoverHighlightView*>(provider.first)->hover()) {
@@ skipping 154 matching lines @@
     }
   }
 
   // Add providers without any configured networks, in the order that the
   // providers were returned by the extensions system.
   for (const VPNProvider& provider : providers)
     AddProviderAndNetworks(provider.key, provider.name, networks);
 }
 
 }  // namespace ash