xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow"
        Height="350"
        Width="525"
        xmlns:igDP="http://infragistics.com/DataPresenter">
   
                    DataSource="{Binding}"/>
   

using System.Windows;
using System.Collections.ObjectModel;

namespace DragDropRecordsInHierarchy
{
    ///
    /// Interaction logic for MainWindow.xaml
    ///
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

            Data = new ObservableCollection
            {
                new Parent
                {
                    ID = 1,
                    Name = "Parent 1",
                    Children1 = new ObservableCollection
                    {
                        new Child { Age = 15, Name = "Child 1", ParentName = "Parnet 1" },
                        new Child { Age = 11, Name = "Child 2", ParentName = "Parnet 1" },
                        new Child { Age = 2, Name = "Child 3", ParentName = "Parnet 1" },
                    },

                    Children2 = new ObservableCollection
                    {
                        new Child { Age = 15, Name = "Child 1", ParentName = "Parnet 1" },
                        new Child { Age = 11, Name = "Child 2", ParentName = "Parnet 1" },
                        new Child { Age = 2, Name = "Child 3", ParentName = "Parnet 1" },
                    }
                },
                new Parent
                {
                    ID = 2,
                    Name = "Parent 2",
                    Children1 = new ObservableCollection
                    {
                        new Child { Age = 7, Name = "Child 1", ParentName = "Parnet 2" },
                        new Child { Age = 1, Name = "Child 2", ParentName = "Parnet 2" },
                        new Child { Age = 4, Name = "Child 3", ParentName = "Parnet 2" },
                    },

                    Children2 = new ObservableCollection
                    {
                        new Child { Age = 15, Name = "Child 1", ParentName = "Parnet 1" },
                        new Child { Age = 11, Name = "Child 2", ParentName = "Parnet 1" },
                        new Child { Age = 2, Name = "Child 3", ParentName = "Parnet 1" },
                    }
                },
                new Parent
                {
                    ID = 3,
                    Name = "Parent 3",
                    Children1 = new ObservableCollection
                    {
                        new Child { Age = 5, Name = "Child 1", ParentName = "Parnet 3" },
                        new Child { Age = 17, Name = "Child 2", ParentName = "Parnet 3" },
                        new Child { Age = 18, Name = "Child 3", ParentName = "Parnet 3" },
                    },

                    Children2 = new ObservableCollection
                    {
                        new Child { Age = 15, Name = "Child 1", ParentName = "Parnet 1" },
                        new Child { Age = 11, Name = "Child 2", ParentName = "Parnet 1" },
                        new Child { Age = 2, Name = "Child 3", ParentName = "Parnet 1" },
                    }
                }
            };

            DataContext = Data;
        }

        public ObservableCollection Data { get; set; }
    }

    public class Parent
    {
        public int ID { get; set; }
        public string Name { get; set; }
        public ObservableCollection Children1 { get; set; }
        public ObservableCollection Children2 { get; set; }
    }

    public class Child
    {
        public string Name { get; set; }
        public int Age { get; set; }
        public string ParentName { get; set; }
    }
}

The key is the second Child collection.

When you launch the app, and drag either the collapsed Child1, or Child2 headers, a null ref exception is thrown.