Check for valid context in AudioOutputDeviceClient::from.

third_party/WebKit/Source/modules/audio_output_devices/AudioOutputDeviceClient.cpp

 // Copyright 2015 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 "modules/audio_output_devices/AudioOutputDeviceClient.h"
 
 #include "core/dom/Document.h"
 #include "core/dom/ExecutionContext.h"
 #include "core/frame/LocalFrame.h"
 
 namespace blink {
 
 const char* AudioOutputDeviceClient::supplementName() {
   return "AudioOutputDeviceClient";
 }
 
 AudioOutputDeviceClient* AudioOutputDeviceClient::from(
     ExecutionContext* context) {
-  if (!context->isDocument())
+  if (!context || !context->isDocument())
     return nullptr;
 
   const Document* document = toDocument(context);
   if (!document->frame())
     return nullptr;
 
   return static_cast<AudioOutputDeviceClient*>(
       Supplement<LocalFrame>::from(document->frame(), supplementName()));
 }
 
 void provideAudioOutputDeviceClientTo(LocalFrame& frame,
                                       AudioOutputDeviceClient* client) {
   frame.provideSupplement(AudioOutputDeviceClient::supplementName(), client);
 }
 
 DEFINE_TRACE(AudioOutputDeviceClient) {
   Supplement<LocalFrame>::trace(visitor);
 }
 
 }  // namespace blink