DataGrid edition 'EditItem' is not allowed for this view` when bound to a WPF DataGrid

Question

I've been reading about this at least for 4 hours, and seems to be the list type, but I have a situation:

A ObservableCollection that has a collection property.

I define the first DataGrid, and in the section

<DataGrid.RowDetailsTemplate>
    <DataTemplate>
        <!-- second Datagrid here, binding to Level2 property of my Observable collection -->
    </DataTemplate>
<DataGrid.RowDetailsTemplate>

Everything goes fine, all things on screen as I expected... but:

1. If try to modify DataGrid1 cells it allow me.
2. If try to modify DataGrid2 cells it throw me this exception 'EditItem' is not allowed for this view

What am I missing ?

This is my model:

public partial class Level1
{
    public Level1()
    {
        this.Level2 = new HashSet<Level2>();
    }

    public decimal IdLevel1 { get; set; }
    public decimal IdLevel2 { get; set; }
    public string StrDescripcionTipoAsociado {get;set;}

    public virtual Level2 Level2{ get; set; }
}

public partial class Level2
{
    public decimal IdLevel1 { get; set; }
    public decimal IdLevel3 { get; set; }

    public virtual Level3 Level3{ get; set; }
}

public partial class Level3
{
    public decimal IdLevel3 { get; set; }
    public decimal NumIdConcepto {get;set;}
    public string StrDescripcionConcepto {get;set;}
}

EDIT: XAML Code:

    <DataGrid Grid.Row="1" 
              ItemsSource="{Binding Level1}" 
              AutoGenerateColumns="False" 
              SelectionMode="Single"
              GridLinesVisibility="Vertical"
              CanUserAddRows="True"
              CanUserDeleteRows="True"
              x:Name="GridTipoAsociado">
        <DataGrid.Columns>
            <DataGridTemplateColumn Header="Tipo de asociado" x:Name="TipoUsuarioSeleccionado">
                <DataGridTemplateColumn.CellTemplate>
                    <DataTemplate>
                        <Label Style="{StaticResource ResourceKey=FontElemNivel1}" Content="{Binding StrDescripcionTipoAsociado}"/>
                    </DataTemplate>
                </DataGridTemplateColumn.CellTemplate>
                <DataGridTemplateColumn.CellEditingTemplate>
                    <DataTemplate>
                        <TextBox Style="{StaticResource ResourceKey=FontElemNivel2}" Text="{Binding StrDescripcionTipoAsociado }"/>
                    </DataTemplate>
                </DataGridTemplateColumn.CellEditingTemplate>
            </DataGridTemplateColumn>
        </DataGrid.Columns>
        <DataGrid.RowDetailsTemplate>
            <DataTemplate>
                <DataGrid Grid.Row="1" 
                          ItemsSource="{Binding Level2}" 
                          AutoGenerateColumns="False" 
                          SelectionMode="Single"
                          SelectionUnit="Cell"
                          GridLinesVisibility="Vertical"
                          CanUserAddRows="True"
                          CanUserDeleteRows="True"                            
                          x:Name="GridItems">
                    <DataGrid.Columns>
                        <DataGridTemplateColumn Header="Id Item">
                            <DataGridTemplateColumn.CellTemplate>
                                <DataTemplate>
                                    <Label Content="{Binding NumIdConcepto}"/>
                                </DataTemplate>
                            </DataGridTemplateColumn.CellTemplate>
                        </DataGridTemplateColumn>
                        <DataGridTemplateColumn Header="Items">
                            <DataGridTemplateColumn.CellTemplate>
                                <DataTemplate>
                                    <Label Content="{Binding Level3.StrDescripcionConcepto}"/>
                                </DataTemplate>
                            </DataGridTemplateColumn.CellTemplate>
                            <DataGridTemplateColumn.CellEditingTemplate>
                                <DataTemplate>
                                    <TextBox Text="{Binding Level3.StrDescripcionConcepto}"/>
                                </DataTemplate>
                            </DataGridTemplateColumn.CellEditingTemplate>
                        </DataGridTemplateColumn>
                    </DataGrid.Columns>
                </DataGrid>
            </DataTemplate>
        </DataGrid.RowDetailsTemplate>
    </DataGrid>

Answer 1

Tks to @nit who give me the right path. Of course the problem reside on the base Type of EF collections

Hashet< T > And Datagrid need at least a List< T >, changing all my classes "those generated by Entity framework", give to me another problem, must make changes manually, and I have a lot of them.

My solution was to create a converter, that made the dirty work for me:

public class listToObservableCollection : BaseConverter, IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        HashSet<Level2> observableList = (HashSet<Level2>)value;
        return new ObservableCollection<Level2>(observableList);
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        return (HashSet<Level2>)value;
    }
}

public abstract class BaseConverter : MarkupExtension
{
    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        return this;
    }
}

And put it on the binding of my Datagrid2:

<!--part of my window definition--!>
xmlns:l="clr-namespace:Recursos;assembly=Recursos"
...
<!--part of my resources section--!>
<l:listToObservableCollection x:Key="listoToObservable"/>
...
<!--part of my datagrid definition--!>
ItemsSource="{Binding Level2,Converter={StaticResource listoToObservable}}"

The only thing on the air is how to make a generic converter, but for now it works fine.