How Do I Remove The Current Usercontrol And Add A New One To A Mainwindow Grid

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WPF

user-controls

C#5.0

A little Background.. I am trying to access the MainWindow's Grid of which my UserControls are being shown.. So i've been able to add and switch between UserControls in my application. HOWEVER after further development i realised an error:

private void ChangeToSecondUserControl()
          {
            SecondUserControl sC = new SecondUserControl();
            var _Main = new MainWindow();
            var Grid = _Main.grdContent;
            int Count = ((Grid)Parent).Children.Count;
            if(Count == 1)
            {
                //int gridIndex = ((Grid)Parent).Children.IndexOf(this);
                //((Grid)Parent).Children.RemoveAt(gridIndex);
                ((Grid)Parent).Children.Add(sC);
            }
        }  

This is the method in which i'm using to traverse between my UserControls. What i discovered was that as i was experimenting, i hadn't been removing the initial UserControl (meaning the second one would load.. however the initial would be directly underneath it)

When i uncomment the commented code, it gives me a NullReference error when it comes to ((Grid)Parent.Children.Add(sC); Could someone explain to me why this is happening?

Note: Even if i have the single line

((Grid)Parent).Children.Remove(0);

(meaning the first child) it gives me an error on that line.. Even if i use

((Grid)Parent).Children.Clear();

Again, when it gets to

((Grid)Parent.Children.Add(sC);

It causes the NullReference Exception again.