cros: AutomationManagerAura::SendEvent crash fix

chrome/browser/ui/aura/accessibility/automation_manager_aura.cc

 // Copyright 2014 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/ui/aura/accessibility/automation_manager_aura.h"
 
 #include <stddef.h>
 
 #include "base/memory/singleton.h"
 #include "build/build_config.h"
@@ skipping 114 matching lines @@
   if (!current_tree_)
     current_tree_.reset(new AXTreeSourceAura());
   reset_serializer ? current_tree_serializer_.reset()
                    : current_tree_serializer_.reset(
                          new AuraAXTreeSerializer(current_tree_.get()));
 }
 
 void AutomationManagerAura::SendEvent(BrowserContext* context,
                                       views::AXAuraObjWrapper* aura_obj,
                                       ui::AXEvent event_type) {
+  if (!current_tree_serializer_)
+    return;
+
   if (!context && g_browser_process->profile_manager()) {
     context = g_browser_process->profile_manager()->GetLastUsedProfile();
   }
 
   if (!context) {
     LOG(WARNING) << "Accessibility notification but no browser context";
     return;
   }
 
   if (processing_events_) {
@@ skipping 80 matching lines @@
     }
   }
 
   // Otherwise, fire the event directly on the Window.
   views::AXAuraObjWrapper* window_wrapper =
       views::AXAuraObjCache::GetInstance()->GetOrCreate(window);
   if (window_wrapper)
     SendEvent(nullptr, window_wrapper, action.hit_test_event_to_fire);
 #endif
 }