WPF数据网格:ItemsSource正在使用时操作无效

请参阅下面的XAML：基本上我正在尝试创建一个分层数据网格。

<Window x:Class="My.Forms.Wpf.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:Asst="clr-namespace:mY.Data;assembly=Assette.Data"
    Title="MainWindow" Height="350" Width="525">
<Window.Resources>
    <ObjectDataProvider x:Key="src" ObjectType="{x:Type Asst:CustomerRepo}" ></ObjectDataProvider>
</Window.Resources>
<Grid DataContext="{StaticResource src}">
    <DataGrid ItemsSource="{Binding Customers}" AutoGenerateColumns="False" RowDetailsVisibilityMode="Collapsed" >
        <DataGrid.Columns>
            <DataGridTextColumn Header="First Name" Binding="{Binding FirstName}" />
            <DataGridTextColumn Header="Last Name" Binding="{Binding LastName}" />
            <DataGridTextColumn Header="Email" Binding="{Binding Email}" />
            <DataGridTextColumn Header="Telephone" Binding="{Binding Telephone}" />
            <DataGridTemplateColumn>
                <DataGridTemplateColumn.CellTemplate>
                    <DataTemplate>
                        <Button Click="ShowHideDetails">Details</Button>
                    </DataTemplate>
                </DataGridTemplateColumn.CellTemplate>
            </DataGridTemplateColumn>
        </DataGrid.Columns>
        <DataGrid.RowDetailsTemplate>
            <DataTemplate>
                <DataGrid ItemsSource="{Binding Orders}" AutoGenerateColumns="False">
                    <DataGridTextColumn Header="Product Name" Binding="{Binding ProductName}" />
                    <DataGridTextColumn Header="Quantity" Binding="{Binding Quantity}" />
                    <DataGridTextColumn Header="Unit Price" Binding="{Binding UnitPrice}" />
                    <DataGridTextColumn Header="Total Price" Binding="{Binding TotalPrice}" />
                </DataGrid>
            </DataTemplate>
        </DataGrid.RowDetailsTemplate>
    </DataGrid>
</Grid>

这绑定到内存中的集合：

   public class CustomerRepo
{
    public ObservableCollection<Customer> Customers { get; set; }
    //CustomerDataEntities custDb = new CustomerDataEntities();
    public CustomerRepo()
    {
        Customers = new ObservableCollection<Customer>();
        Customers.Add(new Customer { FirstName = "John", LastName = "Doe", Email = "aneef@xyz.com", Telephone = "+94773260789" });
        Customers.Add(new Customer { FirstName = "Albert", LastName = "Einstein", Email = "eienstain@xyz.com", Telephone = "+46546546" });
        Customers.Add(new Customer { FirstName = "Carlos", LastName = "Puyol", Email = "puyol@barca.com", Telephone = "+5465465" });

        var customer1 = new Customer { FirstName = "Lionel", LastName = "Messi", Email = "messi@barca.com", Telephone = "+756454686" };
        customer1.Orders.Add(new Order { ProductName = "Boot", Quantity = 1, UnitPrice = 100, TotalPrice = 200 });
        customer1.Orders.Add(new Order { ProductName = "Soccer Ball", Quantity = 1, UnitPrice = 100, TotalPrice = 200 });
        customer1.Orders.Add(new Order { ProductName = "Jersey", Quantity = 1, UnitPrice = 100, TotalPrice = 200 });
        Customers.Add(customer1);
        var customer2 = new Customer { FirstName = "Gerath", LastName = "Bale", Email = "bale@madrid.com", Telephone = "+12312333" };
        customer2.Orders.Add(new Order { ProductName = "Shorts", Quantity = 1, UnitPrice = 100, TotalPrice = 200 });
        customer2.Orders.Add(new Order { ProductName = "Guards", Quantity = 1, UnitPrice = 100, TotalPrice = 200 });
        customer2.Orders.Add(new Order { ProductName = "Sports Bag", Quantity = 1, UnitPrice = 100, TotalPrice = 200 });
        Customers.Add(customer2);
        Customers.Add(new Customer { FirstName = "Cristiano", LastName = "Ronaldo", Email = "ronaldo@madrid.com", Telephone = "+234234234" });
        Customers.Add(new Customer { FirstName = "Iker", LastName = "Cassilas", Email = "cassilas@madrid.com", Telephone = "+54645645" });
    }
}

基本上，当我点击详细信息按钮时，我希望它显示另一个网格的详细信息视图。如果详细信息网格设置为AutoGenerateColumns=true，则一切正常，但当我手动设置列详细信息时，会引发以下异常：

ItemsSource正在使用时操作无效。访问和修改元素替换为ItemsControl.ItemsSource。

你知道吗？

<DataGrid ItemsSource="{Binding Orders}" AutoGenerateColumns="False">
  <DataGrid.Columns> <-- This is missing.
     <DataGridTextColumn Header="Product Name" Binding="{Binding ProductName}" />
     <DataGridTextColumn Header="Quantity" Binding="{Binding Quantity}" />
     <DataGridTextColumn Header="Unit Price" Binding="{Binding UnitPrice}" />
     <DataGridTextColumn Header="Total Price" Binding="{Binding TotalPrice}" />
  </DataGrid.Columns>
</DataGrid>