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

chrome/browser/chromeos/accessibility/accessibility_manager.cc

Index: chrome/browser/chromeos/accessibility/accessibility_manager.cc
diff --git a/chrome/browser/chromeos/accessibility/accessibility_manager.cc b/chrome/browser/chromeos/accessibility/accessibility_manager.cc
index cdf37a793117fb182481dafa6f2c0c9d0ecefed3..fc08dd2ce1d0707436a3524de1517ebe2142f1de 100644
--- a/chrome/browser/chromeos/accessibility/accessibility_manager.cc
+++ b/chrome/browser/chromeos/accessibility/accessibility_manager.cc
@@ -1426,9 +1426,14 @@ 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() {