WPF

private void AddPOItemExecuted()
{
    _PurchaseOrderDialogBase vm = null;

    // Create a new default PO item
    var poItem = new PurchaseOrderItemEntity
    {
        PurchaseOrderHeaderId = PurchaseOrderHeader.Id,
        JobId = PurchaseOrderHeader.JobId,
        PurchaseOrderKind = PurchaseOrderHeader.PurchaseOrderKind
    };

    // Argument class
    var args = new AddEditPOItemArgsModel
    {
        LocationId = SelectedLocation.Id,
        POItem = poItem,
        SequenceSheets = _sequenceSheets,
        POType = PurchaseOrderHeader.PurchaseOrderType
    };

    switch (PurchaseOrderHeader.PurchaseOrderType)
    {
        // Add Equipment to the PO
        case Entities.Enums.PurchaseOrderType.Equipment:
            if (SelectedVendor.Id == AppCore.DNACompany.Id ||
                SelectedVendor.Id == AppCore.ParagonCompany.Id)
            {
                vm = new AddEditEquipmentFromInventoryViewModel(args, "Add Equipment Item");
            }
            else
            {
                vm = new AddEditPOItemViewModel(args, "Add Equipment Item", SelectedVendor.Id);
            }
            break;

        // Add Lumber to the PO
        case Entities.Enums.PurchaseOrderType.Lumber:
            if (SelectedVendor.Id == AppCore.DNACompany.Id ||
                SelectedVendor.Id == AppCore.ParagonCompany.Id)
            {
                vm = new AddEditLumberFromInventoryViewModel(args, "Add Lumber Item");
            }
            else
            {
                vm = new AddEditLumberPOItemViewModel(args, "Add Lumber Item");
            }
            break;

        // OTHER CASE STATEMENTS OMMITED
    }

    if (vm != null)
    {
        DialogResultEx dialogResult = DialogService.ShowDialog(vm, typeof(MainWindowView));

        if ((bool)dialogResult.DialogResult)
        {
            // For Equipment and Tools, only add one line item
            if (PurchaseOrderHeader.PurchaseOrderType == Entities.Enums.PurchaseOrderType.Equipment ||
                PurchaseOrderHeader.PurchaseOrderType == Entities.Enums.PurchaseOrderType.Tools)
            {
                poItem.EntityState = EntityState.New;
                PurchaseOrderHeader.PurchaseOrderItems.Add(poItem);
            }
            else
            {
                // For all other types, add one line item for each selected Lot
                var selectedSheets = vm.SequenceSheetInfos.Where(x => x.IsSelected).ToList();
                foreach (var selectedSheet in selectedSheets)
                {
                    // Clone the line item
                    var newItem = poItem.Clone();    //<============= THROWS HERE

                    // Adjust properties
                    newItem.JobSequenceSheetId = selectedSheet.JobSequenceSheetId;
                    newItem.EntityState = EntityState.New;
                    newItem.PlanElevation = selectedSheet.JobLotInfo;

                    // Add the Line Item to the PO
                    PurchaseOrderHeader.PurchaseOrderItems.Add(newItem);
                }
            }
        }
    }
}

public static T Clone<T>(this T source)
{
    if (!typeof(T).IsSerializable)
    {
        throw new ArgumentException("The type must be serializable.", nameof(source));
    }

    if (ReferenceEquals(source, null))
    {
        return default;
    }

    using (Stream stream = new MemoryStream())
    {
        IFormatter formatter = new BinaryFormatter();
        formatter.Serialize(stream, source);
        stream.Seek(0, SeekOrigin.Begin);
        return (T)formatter.Deserialize(stream);
    }
}

<DataGridTemplateColumn Header="Actions" 
                        Width="220">

<pre>
<DataGridTemplateColumn.CellTemplate>

    <DataTemplate>

        <ComboBox Grid.Row="0"
                    Grid.Column="1"
                    ItemsSource="{Binding Source={StaticResource actions}}"
                    Style="{StaticResource comboBoxStyle}"
                    IsEnabled="True"
                    Width="200"
                    Margin="2">

            <ComboBox.ItemTemplate>
                <DataTemplate>

                    <TextBlock>

                        <Hyperlink Style="{StaticResource HyperlinkStyle}">

                            <TextBlock Text="{Binding Converter={StaticResource enumDescConverter}}"
                                        FontSize="14"/>

                                <i:Interaction.Triggers>
                                    <i:EventTrigger EventName="Click">
                                        <i:InvokeCommandAction Command="{Binding ElementName=dispatchControl, Path=DataContext.ExecuteActionCommand}"
                                                                CommandParameter="{Binding}"/>
                                    </i:EventTrigger>
                                </i:Interaction.Triggers>

                        </Hyperlink>

                    </TextBlock>

                </DataTemplate>

            </ComboBox.ItemTemplate>

        </ComboBox>

    </DataTemplate>

</DataGridTemplateColumn.CellTemplate>

<DataGrid x:Name="seqSheetDataGrid"
          Grid.Row="1"
          Grid.Column="0"
          ItemsSource="{Binding SequenceSheets, Mode=TwoWay}"
          SelectedItem="{Binding SelectedSequenceSheet, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
          RowDetailsVisibilityMode="Visible"
          behaviors:SortDescriptionBehavior.Property="SeqSheetGridSortDescription"
          MaxWidth="1700">

    <DataGrid.Resources>
        <Style TargetType="DataGridCell" BasedOn="{StaticResource dataGridCellStyle}">
            <Style.Triggers>
                <DataTrigger Binding="{Binding Status}" Value="Removed">
                    <Setter Property="FontStyle" Value="Italic" />
                    <Setter Property="Foreground" Value="LightGray" />
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </DataGrid.Resources>

    <DataGrid.RowStyle>
        <Style TargetType="DataGridRow"
               BasedOn="{StaticResource dataGridRowStyle}">
            <Setter Property="BorderBrush" Value="SteelBlue" />
            <Setter Property="BorderThickness" Value="0,0,0,1" />
            <Setter Property="Height" Value="{x:Static sys:Double.NaN}"/>
        </Style>
    </DataGrid.RowStyle>

    <DataGrid.Style>
        <Style TargetType="DataGrid"
               BasedOn="{StaticResource dataGridStyle}">

            <Setter Property="IsReadOnly" Value="True" />
            <Style.Triggers>
                <MultiDataTrigger>
                    <MultiDataTrigger.Conditions>
                        <Condition Binding="{Binding AreFieldsEnabled}" Value="True" />
                        <Condition Binding="{Binding Job.ContractDate, Converter={StaticResource IsNotNullConverter}}" Value="True" />
                        <Condition Binding="{Binding Job.ContractDateInitials, Converter={StaticResource IsNotNullConverter}}" Value="True" />
                    </MultiDataTrigger.Conditions>

                    <MultiDataTrigger.Setters>
                        <Setter Property="IsReadOnly" Value="False" />
                    </MultiDataTrigger.Setters>
                </MultiDataTrigger>
            </Style.Triggers>
        </Style>
    </DataGrid.Style>

    <DataGrid.Columns>

        <DataGridTextColumn Header="Sequence"
                            Binding="{Binding Sequence, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
                            Width="120"
                            SortDirection="Ascending"
                            SortMemberPath="Sequence"
                            IsReadOnly="True"/>

        <DataGridTextColumn Header="Lot's / Bldgs"
                            Binding="{Binding Lot, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
                            Width="120"
                            SortMemberPath="Lot"
                            IsReadOnly="True"/>

        <DataGridTextColumn Header="Plan"
                            Binding="{Binding Plan, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
                            Width="120"
                            SortMemberPath="Plan"
                            IsReadOnly="True"/>

        <DataGridTextColumn Header="Square Feet"
                            Binding="{Binding SquareFeet, StringFormat={}{0:#,#}, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
                            Width="120"
                            HeaderStyle="{StaticResource dataGridRightAlignColHeaderStyle}"
                            SortMemberPath="SquareFeet"
                            IsReadOnly="True">

            <DataGridTextColumn.ElementStyle>
                <Style>
                    <Setter Property="TextBlock.TextAlignment" Value="Right" />
                </Style>
            </DataGridTextColumn.ElementStyle>

        </DataGridTextColumn>

        <DataGridTextColumn Header="Elevation"
                            Binding="{Binding Elevation, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
                            Width="120"
                            SortMemberPath="Elevation"
                            IsReadOnly="True"/>

        <DataGridComboBoxColumn Header="Garage Type"
                                SelectedValueBinding="{Binding GarageTypeId}"
                                SelectedValuePath="Id"
                                DisplayMemberPath="Caption"
                                Width="120"
                                SortMemberPath="GarageTypeId"
                                IsReadOnly="True">

            <DataGridComboBoxColumn.ElementStyle>
                <Style TargetType="{x:Type ComboBox}">
                    <Setter Property="ItemsSource" Value="{Binding ElementName=jobView, Path=DataContext.GarageTypes}" />
                </Style>
            </DataGridComboBoxColumn.ElementStyle>
            <DataGridComboBoxColumn.EditingElementStyle>
                <Style TargetType="{x:Type ComboBox}">
                    <Setter Property="ItemsSource" Value="{Binding ElementName=jobView, Path=DataContext.GarageTypes}" />
                </Style>
            </DataGridComboBoxColumn.EditingElementStyle>

        </DataGridComboBoxColumn>

        <DataGridTextColumn Header="Enhanced Rear"
                            Binding="{Binding EnhancedRear, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, Converter={StaticResource TrueFalseYesNoConverter}}"
                            Width="120"
                            SortMemberPath="EnhancedRear"
                            IsReadOnly="True"/>

        <DataGridTextColumn Header="Enhanced Left"
                            Binding="{Binding EnhancedLeft, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, Converter={StaticResource TrueFalseYesNoConverter}}"
                            Width="120"
                            SortMemberPath="EnhancedLeft"
                            IsReadOnly="True"/>

        <DataGridTextColumn Header="Enhanced Right"
                            Binding="{Binding EnhancedRight, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, Converter={StaticResource TrueFalseYesNoConverter}}"
                            Width="120"
                            SortMemberPath="EnhancedRight"
                            IsReadOnly="True"/>

        <DataGridTextColumn Header="Address"
                            Binding="{Binding Address, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
                            Width="400"
                            SortMemberPath="Address"
                            IsReadOnly="True"/>

        <DataGridTextColumn Header="Status"
                            Binding="{Binding Status, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
                            Width="150"
                            SortMemberPath="Status"
                            IsReadOnly="True"/>

    </DataGrid.Columns>

    <DataGrid.RowDetailsTemplate>
        <DataTemplate>

            <StackPanel Orientation="Horizontal">

                <TextBlock Text="Options: "
                           FontWeight="Bold"
                           Margin="2" />

                <TextBlock Text="{Binding OptionsText}"
                           TextWrapping="Wrap"
                           Margin="2" >
                    <TextBlock.Style>
                        <Style TargetType="TextBlock">
                            <Style.Triggers>
                                <DataTrigger Binding="{Binding Status}" Value="Removed">
                                    <Setter Property="FontStyle" Value="Italic" />
                                    <Setter Property="Foreground" Value="LightGray" />
                                </DataTrigger>
                            </Style.Triggers>
                        </Style>
                    </TextBlock.Style>
                </TextBlock>

            </StackPanel>

        </DataTemplate>

    </DataGrid.RowDetailsTemplate>

</DataGrid>

var callingAssembly = Assembly.GetCallingAssembly();

public MyDialog()
{
    var window = //??? Get the calling assembly's Application.Current.MainWindow

    this.Owner = window;
}