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

chrome/browser/chromeos/accessibility/chromevox_panel.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/shelf/shelf.h"
 #include "ash/shelf/shelf_layout_manager.h"
 #include "ash/shell.h"
 #include "ash/shell_window_ids.h"
 #include "base/macros.h"
 #include "chrome/browser/chromeos/accessibility/accessibility_manager.h"
@@ skipping 91 matching lines @@
 aura::Window* ChromeVoxPanel::GetRootWindow() {
   return GetWidget()->GetNativeWindow()->GetRootWindow();
 }
 
 void ChromeVoxPanel::Close() {
   widget_->Close();
 }
 
 void ChromeVoxPanel::DidFirstVisuallyNonEmptyPaint() {
   widget_->Show();
-  ash::Shelf::ForWindow(GetRootWindow())
-      ->shelf_layout_manager()
-      ->SetChromeVoxPanelHeight(kPanelHeight);
+  UpdatePanelHeight();
+}
+
+void ChromeVoxPanel::UpdatePanelHeight() {
+  ash::Shelf* shelf = ash::Shelf::ForWindow(GetRootWindow());
+  if (!shelf)
+    return;
+
+  ash::ShelfLayoutManager* shelf_layout_manager = shelf->shelf_layout_manager();
+  if (shelf_layout_manager)
+    shelf_layout_manager->SetChromeVoxPanelHeight(kPanelHeight);
 }
 
 void ChromeVoxPanel::EnterFullscreen() {
   Focus();
   fullscreen_ = true;
   UpdateWidgetBounds();
 }
 
 void ChromeVoxPanel::ExitFullscreen() {
   widget_->widget_delegate()->set_can_activate(false);
@@ skipping 32 matching lines @@
                                              uint32_t changed_metrics) {
   UpdateWidgetBounds();
 }
 
 void ChromeVoxPanel::UpdateWidgetBounds() {
   gfx::Rect bounds(GetRootWindow()->bounds().size());
   if (!fullscreen_)
     bounds.set_height(kPanelHeight);
   widget_->SetBounds(bounds);
 }