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

chrome/browser/chromeos/accessibility/chromevox_panel.cc

Index: chrome/browser/chromeos/accessibility/chromevox_panel.cc
diff --git a/chrome/browser/chromeos/accessibility/chromevox_panel.cc b/chrome/browser/chromeos/accessibility/chromevox_panel.cc
index e716e66d4184013672d32e8f560ecbeea0ddd3b9..aff78c5b3d39bc0b346de91d1b4c352e1d3cefb3 100644
--- a/chrome/browser/chromeos/accessibility/chromevox_panel.cc
+++ b/chrome/browser/chromeos/accessibility/chromevox_panel.cc
@@ -109,9 +109,17 @@ void ChromeVoxPanel::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() {