After Selecting Listview App is Crashed

Select Item After Adding List Item, then App is Chrashed
What can i do plz help




Code Behind
List ss;
public AddMenuPage()
{
InitializeComponent();
ss =new List() { "one", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine" };
SelectionListView.ItemsSource = ss;
}

    private void Button_Clicked(object sender, EventArgs e)
    {
        ss.Add("Ten");     
        SelectionListView.ItemsSource = ss;
    }