Fix a crash during extension installation (debug only)....

chrome/browser/extensions/external_pref_extension_provider.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/external_pref_extension_provider.h"
 
 #include "base/file_path.h"
 #include "base/string_util.h"
 #include "base/version.h"
 
@@ skipping 48 matching lines @@
     scoped_ptr<Version> version;
     version.reset(Version::GetVersionFromString(external_version));
     visitor->OnExternalExtensionFound(
         WideToASCII(extension_id), version.get(), FilePath(external_crx));
   }
 }
 
 Version* ExternalPrefExtensionProvider::RegisteredVersion(
     std::string id, Extension::Location* location) const {
   DictionaryValue* extension = NULL;
-  if (!prefs_->GetDictionary(ASCIIToWide(id), &extension)) {
-    NOTREACHED() << "Cannot read extension " << id.c_str()
-                 << " from dictionary.";
+  if (!prefs_->GetDictionary(ASCIIToWide(id), &extension))
     return NULL;
-  }
 
   std::string external_version;
   if (!extension->GetString(kExternalVersion, &external_version))
     return NULL;
 
   if (location)
     *location = Extension::EXTERNAL_PREF;
   return Version::GetVersionFromString(external_version);
 }