Accessibility manager shouldn't try to access ash Shelf from login screen.

chrome/browser/chromeos/accessibility/accessibility_manager.cc

 // Copyright (c) 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 "chrome/browser/chromeos/accessibility/accessibility_manager.h"
 
 #include <stddef.h>
 #include <stdint.h>
 #include <utility>
 
@@ skipping 1408 matching lines @@
   ash::PlaySystemSoundAlways(SOUND_SPOKEN_FEEDBACK_DISABLED);
   // Clear the accessibility focus ring.
   AccessibilityFocusRingController::GetInstance()->SetFocusRing(
       std::vector<gfx::Rect>());
 }
 
 void AccessibilityManager::OnChromeVoxPanelClosing() {
   aura::Window* root_window = chromevox_panel_->GetRootWindow();
   chromevox_panel_widget_observer_.reset(nullptr);
   chromevox_panel_ = nullptr;
-  ash::Shelf::ForWindow(root_window)
-      ->shelf_layout_manager()
-      ->SetChromeVoxPanelHeight(0);
+
+  ash::Shelf* shelf = ash::Shelf::ForWindow(root_window);
+  if (!shelf)
+    return;
+
+  ash::ShelfLayoutManager* shelf_layout_manager = shelf->shelf_layout_manager();
+  if (shelf_layout_manager)
+    shelf_layout_manager->SetChromeVoxPanelHeight(0);
 }
 
 void AccessibilityManager::OnChromeVoxPanelDestroying() {
   chromevox_panel_widget_observer_.reset(nullptr);
   chromevox_panel_ = nullptr;
 }
 
 void AccessibilityManager::SetKeyboardListenerExtensionId(
     const std::string& id,
     content::BrowserContext* context) {
   keyboard_listener_extension_id_ = id;
 
   extensions::ExtensionRegistry* registry =
       extensions::ExtensionRegistry::Get(context);
   if (!extension_registry_observer_.IsObserving(registry) && !id.empty())
     extension_registry_observer_.Add(registry);
 }
 
 }  // namespace chromeos