Problem with Workflow Association form in sharepoint 2013 using visual studio 2012

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 here http://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 notset 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 am new to the sharepoint workflow development.Could you please help me