Null check MediaSourceRegistry::lookup

third_party/WebKit/Source/modules/mediasource/MediaSourceRegistry.cpp

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 49 matching lines @@
         return;
 
     MediaSource* source = iter->value;
     m_mediaSources.remove(iter);
     source->removedFromRegistry();
 }
 
 URLRegistrable* MediaSourceRegistry::lookup(const String& url)
 {
     DCHECK(isMainThread());
-    return m_mediaSources.get(url);
+    return url.isNull() ? nullptr : m_mediaSources.get(url);
 }
 
 MediaSourceRegistry::MediaSourceRegistry()
 {
     HTMLMediaSource::setRegistry(this);
 }
 
 } // namespace blink