datagrid inside datagrid with DataGridTemplateColumn.CellEditingTemplate

public class Product
    {
        public int ProductID { get; set; }
        public string Name { get; set; }
    }
    public class Category
    {
        public int CategoryID { get; set; }
        public string Name { get; set; }
        public List<Product> Products { get; set; }
    }

<data:DataGrid x:Name="theGrid"
                       AutoGenerateColumns="True"
                       RowDetailsVisibility="VisibleWhenSelected"
                       HeadersVisibility="All"
                       >
            <data:DataGrid.RowDetailsTemplate>
                <DataTemplate>
                    <data:DataGrid Height="100" ItemsSource="{Binding Products}" AutoGenerateColumns="True" HeadersVisibility="Column"></data:DataGrid>
                </DataTemplate>
            </data:DataGrid.RowDetailsTemplate>
        </data:DataGrid>

List<Category> Categories = new List<Category>();
for (int i = 1; i < 11; i++)
{
    Category cat = new Category { CategoryID = i, Name = "Category" + i };
    cat.Products = new List<Product>();
    for (int j = 1; j < 6; j++)
    {
        Product p = new Product { ProductID = (i * j), Name = "Product " + (i * j).ToString() };
        cat.Products.Add(p);
    }
    Categories.Add(cat);
}


theGrid.ItemsSource = Categories;