Fix DesktopCapturer constructors to handle failures properly.

webrtc/modules/desktop_capture/desktop_capturer.cc

 /*
  *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 20 matching lines @@
 }
 
 bool DesktopCapturer::FocusOnSelectedSource() {
   return false;
 }
 
 // static
 std::unique_ptr<DesktopCapturer> DesktopCapturer::CreateWindowCapturer(
     const DesktopCaptureOptions& options) {
   std::unique_ptr<DesktopCapturer> capturer = CreateRawWindowCapturer(options);
-  if (options.detect_updated_region()) {
+  if (capturer && options.detect_updated_region()) {
     capturer.reset(new DesktopCapturerDifferWrapper(std::move(capturer)));
   }
 
   return capturer;
 }
 
 // static
 std::unique_ptr<DesktopCapturer> DesktopCapturer::CreateScreenCapturer(
     const DesktopCaptureOptions& options) {
   std::unique_ptr<DesktopCapturer> capturer = CreateRawScreenCapturer(options);
-  if (options.detect_updated_region()) {
+  if (capturer && options.detect_updated_region()) {
     capturer.reset(new DesktopCapturerDifferWrapper(std::move(capturer)));
   }
 
   return capturer;
 }
 
 }  // namespace webrtc