User Control inside the Tabitem in wpf using mvvm

I am using WPF MVVM model in my project using C#.Net.

I am trying to use usercontrol inside the tabitem.
When i run the project I cannot access usercontrol. It displays error "Value cannot be null. No parameter element".

I have attached my XAML and ViewModel Code for your reference.

XAML Code:
==========

Header="Applicant Details">


View Model:
==========
WindowElement winElement = GetWindowElement.GetElement(Collection.FormApplicantDetails);
UserControl userControl = Getchild.GetField(Collection.ApplicantUserControl, Collection.ApplicantUserControlName) as

UserControl;
if (DataValidator.IsValid(userControl))
{


}

Please advise.