DiscoverMeasures throws NullReferenceException

HI,
 
I'm using the MVC accelerator "Flexmonster.Accelerator.Controllers.FlexmonsterProxyController".
 
When loading the pivot chart, it cycles through a few discovery steps, (dimensions, hierarchies, levels) but blows up on DiscoverMeasures.
 
The report then displays "Object reference not set to an instance of an object".  It took me a while to expose the underlying stack trace as you are using NLog and I don't use that logger, but I found it. See below. Any suggestion as to what might be the issue?
 

Start your code here2018-07-23 16:47:07.1524|ERROR|FlexmonsterProxy|Object reference not set to an instance of an object.
 at Microsoft.AnalysisServices.HttpStream.ImpersonateLogonIdentityAndExecute(Action action)
 at Microsoft.AnalysisServices.HttpStream.GetResponseStream()
 at Microsoft.AnalysisServices.HttpStream.GetResponseDataType()
 at Microsoft.AnalysisServices.CompressedStream.GetResponseDataType()
 at Microsoft.AnalysisServices.XmlaClient.EndRequest(Boolean useBinaryXml)
 at Microsoft.AnalysisServices.XmlaClient.CreateSession(ListDictionary properties, Boolean sendNamespaceCompatibility, String sessionToken)
 at Microsoft.AnalysisServices.XmlaClient.Connect(ConnectionInfo connectionInfo, Boolean beginSession)
 at Microsoft.AnalysisServices.Core.Server.Connect(String connectionString, String sessionId, ObjectExpansion expansionType)
 at Microsoft.AnalysisServices.Core.Server.Connect(String connectionString)
 at Flexmonster.Accelerator.Controllers.FlexmonsterProxyController.DiscoverMeasures(DiscoverArgs args)