Synfusion WFP Control SfDataGrid null reference exception after changing ItemsSource during runtime

Question

In Synfusion SfDataGrid, I want to replace the ItemsSource during runtime with a new collection. my ItemsSource is an ObservableCollection which I have already implemented the INotifyPropertyChanged requirements for it. The definition of my Collection is as follow:

public ObservableCollection<JxTask> Tasks
        {
            get
            {
                if (_tasks == null)
                {
                    _tasks = MainModel.GetIssues();
                }
                return _tasks;
            }
            set
            {
                if (_tasks != value)
                {
                    _tasks = value;
                    OnPropertyChanged("Tasks");
                }
            }
        }

Drop event function is defined as follow:

private void RowDragDropController_OnRowDropped(object sender, GridRowDroppedEventArgs e)
{
            ObservableCollection<JxTask> aktuelleissues = MainModel.GetIssues();
            ObservableCollection<JxTask> alteIssues = sfDataGrid.ItemsSource as ObservableCollection<JxTask>;

            if (alteIssues != null && aktuelleissues != null)
            {
                foreach (var item in alteIssues)
                {
                    item.sortOrder = (from x in aktuelleissues where x.id == item.id select x).FirstOrDefault().sortOrder;
                }
            }
            if (aktuelleissues != null)
            {
                sfDataGrid.ItemsSource = aktuelleissues;
            }            
            CollectionViewSource.GetDefaultView(sfDataGrid.ItemsSource).Refresh();
}


everything works fine after first drop (grid will get updated with new collection) but as soon as trying to drag another item, i rises **System.NullReferenceException: 'Object reference not set to an instance of an object.'**

Any advice what may cause this?