Hello, 

Thank you for posting to IG forums! 

Here I have attached a sample project representing how drag and drop functionality could be used for changing record’s position in hierarchical XamDataGrid. Could you please run the project on your side and let me know whether you get the same error? What is the specific version of Infragistics assemblies you are using? Thank you.

        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.