MacViews: Fix crash when EventMonitorMac receives a NSSystemDefinedEvent

ui/views/event_monitor_mac.mm

 // 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.
 
 #import "ui/views/event_monitor_mac.h"
 
 #import <Cocoa/Cocoa.h>
 
 #include "base/logging.h"
 #include "ui/events/event.h"
@@ skipping 24 matching lines @@
   return gfx::Point(mouseLocation.x, mouseLocation.y);
 }
 
 EventMonitorMac::EventMonitorMac(ui::EventHandler* event_handler,
                                  gfx::NativeWindow target_window) {
   DCHECK(event_handler);
   monitor_ = [NSEvent addLocalMonitorForEventsMatchingMask:NSAnyEventMask
       handler:^NSEvent*(NSEvent* event) {
           if (!target_window || [event window] == target_window) {
             scoped_ptr<ui::Event> ui_event = ui::EventFromNative(event);
-            event_handler->OnEvent(ui_event.get());
+            if (ui_event)
+              event_handler->OnEvent(ui_event.get());
           }
           return event;
       }];
 }
 
 EventMonitorMac::~EventMonitorMac() {
   [NSEvent removeMonitor:monitor_];
 }
 
 }  // namespace views