System.NullReferenceException

[Floryndas]

Dear,
While trying to build your project Visual Studio 2017, I always face the error System.NullReferenceException HResult=0x80004003 Message=Object reference not set to an instance of an object. Source=NIRAS.Revit.TTL_Exporter Arborescence des appels de procédure : at NIRAS.Revit.TTL_Exporter.Export_TTL_File_Main.Execute(ExternalCommandData commandData, String& message, ElementSet elements) in C:\Users\Justine\source\repos\revit-bot-exporter-master\revit-bot-exporter-master\NIRAS.Revit.TTL_Exporter\NIRAS.Revit.TTL_Exporter.Main.cs:line 59 at apiManagedExecuteCommand(AString* assemblyName, AString* className, AString* vendorDescription, MFCApp* pMFCApp, DBView* pDBView, AString* message, Set<ElementId\,std::less<ElementId>\,tnallc<ElementId> >* ids, Map<AString\,AString\,std::less<AString>\,tnallc<std::pair<AString const \,AString> > >* data, AString* exceptionName, AString* exceptionMessage) at line 59 which is the following:
String Host = (new FilteredElementCollector(doc) .OfCategory(BuiltInCategory.OST_ProjectInformation) .FirstElement() as ProjectInfo).LookupParameter("Host").AsString();

Also, could you give me an example of HOST variable value, please? What does HOST means for a particular BIM project?
Thanks in advance for your return.