Can't activate site feature

Hi guys,

I’ve downloaded the demo for Workflow Actions and deployed successfully onto my SharePoint server.
I managed to activate the web application feature successfully but I can’t activate the site feature. I receive the following error in the ULS log:

Getting Error Message for Exception System.Web.HttpUnhandledException (0x80004005): Exception of type ‘System.Web.HttpUnhandledException’ was thrown. —> System.NullReferenceException: Object reference not set to an instance of an object.
at Microsoft.SharePoint.WorkflowServices.WorkflowServicesContext.GetProperty[T](WorkflowServiceProperty property)
at Microsoft.SharePoint.WorkflowServices.SPWebWorkflowSecurityContext.get_KeyGroupId()
at Microsoft.SharePoint.WorkflowServices.SPWebWorkflowSecurityContext.EnsureAppPrincipal(SPWeb elevatedWeb)
at Microsoft.SharePoint.WorkflowServices.SPWebWorkflowSecurityContext.b__3()


Please ensure that you have correctly configured SharePoint Workflow Services … x#section6