Error when opening up Dynamics Workflow file (Xaml ) in Visual Studio 2010/2012

When I try to open xaml file, the designer only shows a red box with "Could not generate view for Workflow"

Here's the steps I took:

1. create a simple workflow in Dynamics CRM 2011
2. add the workflow to a solution, and exported the solution as unmanaged.
3. In Visual Studio 2010, create a .NET 4 Activity Library
4. Right click on project, and add existing and select the xaml file exported in the solution
5. Double click on the .xaml file to view it in designer

I have also added reference of microsoft.xrm.sdk.dll and microsoft.xrm.sdk.workflow.dll in my project.

Actual exception:

System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.Xrm.Sdk.Workflow.Activities.ReferenceLiteral`1.CacheMetadata(CodeActivityMetadata metadata)
   at System.Activities.CodeActivity`1.OnInternalCacheMetadataExceptResult(Boolean createEmptyBindings)
   at System.Activities.Activity`1.OnInternalCacheMetadata(Boolean createEmptyBindings)
   at System.Activities.Activity.InternalCacheMetadata(Boolean createEmptyBindings, IList`1& validationErrors)
   at System.Activities.ActivityUtilities.ProcessActivity(ChildActivity childActivity, ChildActivity& nextActivity, Stack`1& activitiesRemaining, ActivityCallStack parentChain, IList`1& validationErrors, ProcessActivityTreeOptions options, ProcessActivityCallback callback)
   at System.Activities.ActivityUtilities.ProcessActivityTreeCore(ChildActivity currentActivity, ActivityCallStack parentChain, ProcessActivityTreeOptions options, ProcessActivityCallback callback, IList`1& validationErrors)
   at System.Activities.ActivityUtilities.CacheRootMetadata(Activity activity, LocationReferenceEnvironment hostEnvironment, ProcessActivityTreeOptions options, ProcessActivityCallback callback, IList`1& validationErrors)
   at System.Activities.Validation.ActivityValidationServices.InternalActivityValidationServices.InternalValidate()
   at System.Activities.Validation.ActivityValidationServices.Validate(Activity toValidate, ValidationSettings settings)
   at System.Activities.Presentation.Validation.ValidationService.CoreValidationWork(ValidationReason reason, CancellationToken cancellationToken)
d:\Projects\ActivityLibrary1\ActivityLibrary1\SendEmailProcess-5F3D855D-9B4D-4E3F-8DE0-09622D20E786.xaml

Any Idea??