Check for valid context in AudioOutputDeviceClient::from.

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

Index: third_party/WebKit/Source/modules/audio_output_devices/AudioOutputDeviceClient.cpp
diff --git a/third_party/WebKit/Source/modules/audio_output_devices/AudioOutputDeviceClient.cpp b/third_party/WebKit/Source/modules/audio_output_devices/AudioOutputDeviceClient.cpp
index b271ce3d38014e5f99e43b02e052d27cbd4ad1bb..67f9394a8ea68a0f15ab5891693bba9e185e61f6 100644
--- a/third_party/WebKit/Source/modules/audio_output_devices/AudioOutputDeviceClient.cpp
+++ b/third_party/WebKit/Source/modules/audio_output_devices/AudioOutputDeviceClient.cpp
@@ -16,7 +16,7 @@ const char* AudioOutputDeviceClient::supplementName() {
 
 AudioOutputDeviceClient* AudioOutputDeviceClient::from(
     ExecutionContext* context) {
-  if (!context->isDocument())
+  if (!context || !context->isDocument())
     return nullptr;
 
   const Document* document = toDocument(context);