replacing the NullCulprit class by None

appengine/findit/crash/findit.py

 # Copyright 2016 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.
 
 import copy
 import logging
 
 from google.appengine.ext import ndb
 
 from common import appengine_util
 from common import chrome_dependency_fetcher
 from common import constants
 from common import time_util
 from crash.crash_report import CrashReport
-from crash.culprit import Culprit
 from model import analysis_status
 from model.crash.crash_config import CrashConfig
 
 # TODO(http://crbug.com/659346): since most of our unit tests are
 # FinditForFracas-specific, wrengr moved them to findit_for_chromecrash_test.py.
 # However, now we're missing coverage for most of this file (due to the
 # buggy way coverage is computed). Need to add a bunch of new unittests
 # to get coverage back up.
 
 # TODO: this class depends on ndb stuff, and should therefore move to
@@ skipping 245 matching lines @@
   # TODO(wrengr): This is only called by ``CrashAnalysisPipeline.run``;
   # we should be able to adjust things so that we only need to take in
   # ``crash_identifiers``, or a CrashReport, rather than taking in the
   # whole model. And/or, we should just inline this there.
   # TODO(http://crbug.com/659346): coverage tests for this class, not
   # just for FinditForFracas.
   def FindCulprit(self, model): # pragma: no cover
     """Given a CrashAnalysis ndb.Model, return a Culprit."""
     stacktrace = self.ParseStacktrace(model)
     if stacktrace is None:
-      return Culprit('', [], [], None, None)
+      return None
 
     return self._predator.FindCulprit(CrashReport(
         crashed_version = model.crashed_version,
         signature = model.signature,
         platform = model.platform,
         stacktrace = stacktrace,
         regression_range = model.regression_range))