Fix a crash in the extension system when sending response back

chrome/browser/extensions/extension_function.cc

 // Copyright (c) 2009 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 "chrome/browser/extensions/extension_function.h"
 
 #include "base/json_reader.h"
 #include "base/json_writer.h"
 #include "base/logging.h"
 #include "chrome/browser/extensions/extension_function_dispatcher.h"
 
 void AsyncExtensionFunction::SetArgs(const std::string& args) {
   DCHECK(!args_);  // should only be called once
   if (!args.empty()) {
     JSONReader reader;
     args_ = reader.JsonToValue(args, false, false);
 
     // Since we do the serialization in the v8 extension, we should always get
     // valid JSON.
     if (!args_) {
       DCHECK(false);
       return;
     }
   }
 }
 
 const std::string AsyncExtensionFunction::GetResult() {
   std::string json;
-  JSONWriter::Write(result_.get(), false, &json);
+  // Some functions might not need to return any results.
+  if (result_.get())
+    JSONWriter::Write(result_.get(), false, &json);
   return json;
 }
 
 void AsyncExtensionFunction::SendResponse(bool success) {
   if (bad_message_) {
     dispatcher_->HandleBadMessage(this);
   } else {
     dispatcher_->SendResponse(this, success);
   }
 }
 
 std::string AsyncExtensionFunction::extension_id() {
   return dispatcher_->extension_id();
 }
 
 Profile* AsyncExtensionFunction::profile() {
   return dispatcher_->profile();
 }