The GestureRecognizer should never map an ID to a NULL consumer

ui/events/gestures/gesture_recognizer_impl.cc

 // Copyright (c) 2012 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/events/gestures/gesture_recognizer_impl.h"
 
 #include <limits>
 
 #include "base/command_line.h"
 #include "base/logging.h"
@@ skipping 65 matching lines @@
 
 // Checks if this finger is already down, if so, returns the current target.
 // Otherwise, returns NULL.
 GestureConsumer* GestureRecognizerImpl::GetTouchLockedTarget(
     const TouchEvent& event) {
   return touch_id_target_[event.touch_id()];
 }
 
 GestureConsumer* GestureRecognizerImpl::GetTargetForGestureEvent(
     const GestureEvent& event) {
-  GestureConsumer* target = NULL;
   int touch_id = event.details().oldest_touch_id();
-  target = touch_id_target_for_gestures_[touch_id];
-  return target;
+  if (!touch_id_target_for_gestures_.count(touch_id)) {
+    NOTREACHED() << "Touch ID does not map to a valid GestureConsumer.";
+    return nullptr;
+  }
+
+  return touch_id_target_for_gestures_.at(touch_id);
 }
 
 GestureConsumer* GestureRecognizerImpl::GetTargetForLocation(
     const gfx::PointF& location, int source_device_id) {
   const float max_distance =
       GestureConfiguration::GetInstance()
           ->max_separation_for_gesture_touches_in_pixels();
 
   gfx::PointF closest_point;
   int closest_touch_id = 0;
@@ skipping 239 matching lines @@
   std::vector<GestureEventHelper*>::iterator it;
   for (it = helpers.begin(); it != helpers.end(); ++it)
     gesture_recognizer->AddGestureEventHelper(*it);
 
   helpers.clear();
   g_gesture_recognizer_instance =
       static_cast<GestureRecognizerImpl*>(gesture_recognizer);
 }
 
 }  // namespace ui