Problem with Workflow Association form

Hi,

I created an empty SharePoint 2013 solution, added there worklow, tested that it works. Then added Association form to workflow (I exactly followed the steps from herehttp://msdn.microsoft.com/en-us/library/office/dn518136.aspx).

Now, I deployed it from VS, tried to assign it to the library, but after clicking Next on default page the redirection to my Association form fails: "An unexpected error has occurred". I checked ULS logs using CorreclationID and found the following error:

System.NullReferenceException: Object reference not set to an instance of an object.   
 at Microsoft.SharePoint.Utilities.SPChunkedStringBuffer.Append(String str)    
 at Microsoft.SharePoint.WorkflowServices.ApplicationPages.WorkflowAssociationFormContextControl.OnLoad(EventArgs e)    
 at System.Web.UI.Control.LoadRecursive()    
 at System.Web.UI.Control.LoadRecursive()    
 at System.Web.UI.Control.LoadRecursive()    
 at System.Web.UI.Control.LoadRecursive()    
 at System.Web.UI.Control.LoadRecursive()    
 at System.Web.UI.Control.LoadRecursive()    
 at System.Web.UI.Control.LoadRecursive()    
 at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

I found this thread, but I don't see how can I apply this recipe since I don't have the mentioned reference. I have the following references on AssociationForm.aspx page:

<%@ Page Inherits="Microsoft.SharePoint.WebPartPages.WebPartPage, Microsoft.SharePoint, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" MasterPageFile="~masterurl/default.master" Language="C#" %><%@ Register TagPrefix="Utilities" Namespace="Microsoft.SharePoint.Utilities" Assembly="Microsoft.SharePoint, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %><%@ Register TagPrefix="WebPartPages" Namespace="Microsoft.SharePoint.WebPartPages" Assembly="Microsoft.SharePoint, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %><%@ Register TagPrefix="SharePoint" Namespace="Microsoft.SharePoint.WebControls" Assembly="Microsoft.SharePoint, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %><%@ Register Tagprefix="WorkflowServices" Namespace="Microsoft.SharePoint.WorkflowServices.ApplicationPages" Assembly="Microsoft.SharePoint.WorkflowServices.ApplicationPages, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>