Event error in DownloadDecitionMaker - Object reference not set to an instance of an object

[zogral]

**Sonarr version: 3.0.0.334
**Mono version: 5.16.0.220
**OS: 18.04 LTS
Debug logs:

Message
Couldn't evaluate decision on Westworld.S02E03.BDRip.X264-REWARD English: Object reference not set to an instance of an object

Exception

System.NullReferenceException: Object reference not set to an instance of an object
  at NzbDrone.Core.Profiles.Releases.PreferredWordService.GetMatchingPairs (NzbDrone.Core.Tv.Series series, System.String title, System.Boolean isRenaming) [0x00000] in M:\BuildAgent\work\63739567f01dbcc2\src\NzbDrone.Core\Profiles\Releases\PreferredWordService.cs:50 
  at NzbDrone.Core.Profiles.Releases.PreferredWordService.Calculate (NzbDrone.Core.Tv.Series series, System.String title) [0x00011] in M:\BuildAgent\work\63739567f01dbcc2\src\NzbDrone.Core\Profiles\Releases\PreferredWordService.cs:31 
  at NzbDrone.Core.DecisionEngine.Specifications.RssSync.HistorySpecification.IsSatisfiedBy (NzbDrone.Core.Parser.Model.RemoteEpisode subject, NzbDrone.Core.IndexerSearch.Definitions.SearchCriteriaBase searchCriteria) [0x000ac] in M:\BuildAgent\work\63739567f01dbcc2\src\NzbDrone.Core\DecisionEngine\Specifications\RssSync\HistorySpecification.cs:55 
  at NzbDrone.Core.DecisionEngine.DownloadDecisionMaker.EvaluateSpec (NzbDrone.Core.DecisionEngine.Specifications.IDecisionEngineSpecification spec, NzbDrone.Core.Parser.Model.RemoteEpisode remoteEpisode, NzbDrone.Core.IndexerSearch.Definitions.SearchCriteriaBase searchCriteriaBase) [0x00000] in M:\BuildAgent\work\63739567f01dbcc2\src\NzbDrone.Core\DecisionEngine\DownloadDecisionMaker.cs:152

Description of issue: Error found in events log. Does not happen on every download, but a lot of them.