Do not dispatch gestures to a NULL aura::Window target

ui/aura/window_event_dispatcher.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 "ui/aura/window_event_dispatcher.h"
 
 #include "base/bind.h"
 #include "base/debug/trace_event.h"
 #include "base/logging.h"
 #include "base/message_loop/message_loop.h"
@@ skipping 267 matching lines @@
   return DispatchEvent(mouse_moved_handler_, &translated_event);
 }
 
 ui::EventDispatchDetails WindowEventDispatcher::ProcessGestures(
     ui::GestureRecognizer::Gestures* gestures) {
   DispatchDetails details;
   if (!gestures || gestures->empty())
     return details;
 
   Window* target = GetGestureTarget(gestures->get().at(0));
+  if (!target)
+    return details;
+
   for (size_t i = 0; i < gestures->size(); ++i) {
     ui::GestureEvent* event = gestures->get().at(i);
     event->ConvertLocationToTarget(window(), target);
     details = DispatchEvent(target, event);
     if (details.dispatcher_destroyed || details.target_destroyed)
       break;
   }
   return details;
 }
 
@@ skipping 593 matching lines @@
     if (!ui::GestureRecognizer::Get()->ProcessTouchEventPreDispatch(orig_event,
                                                                     target)) {
       event->SetHandled();
     }
   }
 
   PreDispatchLocatedEvent(target, event);
 }
 
 }  // namespace aura