Null Reference Error when previewing a task form after migrating from 2010

Hi All,

When I attempt to preview a task form edited with Nintex Forms I get the "Server Error in '/' Application" screen.  On the server the event viewer shows the following 1309 Web Event

 Exception information: 

    Exception type: NullReferenceException 

    Exception message: Object reference not set to an instance of an object.

   at Nintex.Forms.SharePoint.ApplicationPages.PreviewNintexForm.GetDefaultValuesForWorkflowVariables()

   at Nintex.Forms.SharePoint.ApplicationPages.PreviewNintexForm.InitializePreviewFiller()

   at Nintex.Forms.SharePoint.ApplicationPages.PreviewNintexForm.OnInit(EventArgs e)

   at System.Web.UI.Control.InitRecursive(Control namingContainer)

   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint

 Seems to only happen on workflows that were built in 2010 and migrated to 2013.  

 Any ideas?

Thanks