The calling thread cannot access the object becuase a different thread owns it

I have a WPF MVVM application.
I have this particular piece of code which gives the above error.

It is triggered whn a button is clicked and works fine the first time the button is clicked.
But the next time arround on the second button click I get the error.
PivotGrid is an UI element in my view, and I am accessing the UI element in my Viw Model

 Application.Current.Dispatcher.Invoke(System.Windows.Threading.DispatcherPriority.Normal, new Action(
                  delegate()
                  {
                      pivotGrid = App.Current.Windows[0].FindName("pivotGrid") as PivotGridControl;

                  }));