NullReferenceException in CatalogFeedJob when no wildcard hostname

Description

Steps to reproduce:

1. Go to a Commerce site > Admin mode > Config tab > Manage Websites > remove the wildcard hostname.
2. Open the Admin tab.
3. Run the Export Product Feed job.

Expected:
No problems.

Actual:
It shows error "Object reference not set to an instance of an object."
The error in the Log file:
2018-09-19 10:45:33,190 [304] ERROR EPiServer.DataAbstraction.ScheduledJob: Job EPiServer.Personalization.Commerce.CatalogFeed.CatalogFeedJob failed for the job 'Export Product Feed' with jobId ='8e740d68-e17e-4737-bb21-de18e1126d1c'
System.NullReferenceException: Object reference not set to an instance of an object.
at EPiServer.Personalization.Commerce.CatalogFeed.DefaultFeedUrlConverter.GetExternalUrl(String relativePath, String scope)
at EPiServer.Personalization.Commerce.CatalogFeed.CatalogFeedCreation.CreateLanguageElements(EntryContentBase entryContent, Decimal stock, String scope)
at EPiServer.Personalization.Commerce.CatalogFeed.CatalogFeedService.AddStockElements(SyndicationItem item, ProductContent productContent, EntryContentBase entryContent, IEnumerable`1 productVariations, IEnumerable`1 languages, String scope)
at EPiServer.Personalization.Commerce.CatalogFeed.CatalogFeedService.GetSyndicationItem(EntryContentBase entryContent, CultureInfo defaultLanguage, IEnumerable`1 languages, String scope)
at EPiServer.Personalization.Commerce.CatalogFeed.CatalogFeedService.<>c_DisplayClass9_0.<GetSyndicationItems>b_1(ProductContent x)
at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
at System.Linq.Enumerable.WhereEnumerableIterator`1.MoveNext()
at EPiServer.Personalization.Commerce.CatalogFeed.CatalogFeedService.<GetSyndicationItems>d__9.MoveNext()
at System.ServiceModel.Syndication.Rss20FeedFormatter.WriteItems(XmlWriter writer, IEnumerable`1 items, Uri feedBaseUri)
at System.ServiceModel.Syndication.Rss20FeedFormatter.WriteFeed(XmlWriter writer)
at System.ServiceModel.Syndication.Rss20FeedFormatter.System.Xml.Serialization.IXmlSerializable.WriteXml(XmlWriter writer)
at EPiServer.Personalization.Commerce.CatalogFeed.CatalogRss20FeedFormatter.WriteTo(XmlWriter writer)
at EPiServer.Personalization.Commerce.CatalogFeed.CatalogFeedJob.Execute()
at EPiServer.Scheduler.Internal.DefaultScheduledJobExecutor.<Execute>d__31.MoveNext()