Workflow Association form for SharePoint 2013 workflow

I've developed a SharePoint 2013 workflow. Visual Studio 2012 provides a Workflow Association form template which I've added in my workflow. The workflow association form template has some HTML syntax errors. Anyway, after deploying my workflow I can't navigate to the association form. As soon as I move to my association page, I get unexpected error. After investigating the log file I've found the following error.

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.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)     
at System.Web.UI.Page.HandleError(Exception e)     
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)     
at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)     
at System.Web.UI.Page.ProcessRequest()     
at System.Web.UI.Page.ProcessRequest(HttpContext context)     
at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()     
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)   

Have any of you tried the Workflow Association form template provided in Visual Studio 2012 for SharePoint 2013 workflow? The template itself has syntax error as well as it doesn't work at all. I've also tried to use custom code behind for the workflow association form but it didn't work either.

Thanks,
Sohel Rana
http://ranaictiu-technicalblog.blogspot.com