Custom XAML Workflow Object reference not set to an instance of an object Visual Studio

Error 1 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)
 C:\Users\rbbmdx\Documents\sdk_5.0.13\sdk\walkthroughs\workflow\xamlworkflowproject\XAMLWorkflowProject\SendContactMail-0727EE18-190F-46B7-A3A9-E212E6DCB1EB.xaml 0 0