public class CustomTreeListView : RadTreeListView
{
	protected override AutomationPeer OnCreateAutomationPeer()
	{
		return new CustomTreeListViewAutomationPeer(this);
	}
}

public class CustomTreeListViewAutomationPeer : RadTreeListViewAutomationPeer
{
	public CustomTreeListViewAutomationPeer(RadTreeListView owner) 
		: base(owner)
	{
		var methodInfo = typeof(RadTreeListViewAutomationPeer).GetMethod("GetItemPeers", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic);
		methodInfo.Invoke(this, null);
	}

	protected override List<AutomationPeer> GetChildrenCore()
	{
		var frameworkPeer = new FrameworkElementAutomationPeer((FrameworkElement)this.Owner);
		return frameworkPeer.GetChildren();
	}
}

    I found a bug when use MergeCells in grid view and tree list view, I am the license account, my support is out of date,But I still want to feadback the bug to you ， I make a vedio in attachment

When use in radtreelistview , I can not explict set the MergedCellsStyle, maybe MergedCellsStyleSelector also have the bug.I just try MergedCellsStyle.

InvalidOperationException
   at System.ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumFailedVersion() in /_/src/libraries/System.Private.CoreLib/src/System/ThrowHelper.cs:line 432
   at System.Collections.Generic.List`1.Enumerator.MoveNext() in /_/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/List.cs:line 1130
   at System.Linq.Enumerable.SelectEnumerableIterator`2.MoveNext()
   at Telerik.Windows.Data.EnumerableExtensions.<SelectRecursive>d__15`1.MoveNext()
   at System.Linq.Enumerable.SelectEnumerableIterator`2.MoveNext()
   at Telerik.Windows.Data.HierarchicalCollectionViewBase.PopulateInternalList(IQueryable view)
   at Telerik.Windows.Data.QueryableCollectionView.get_InternalList()
   at Telerik.Windows.Data.HierarchicalCollectionViewBase.get_InternalCount()
   at Telerik.Windows.Data.DataItemCollection.get_Item(Int32 index)
   at Telerik.Windows.Controls.GridView.Rows.GetRowItemsAtRange(Int32 startIndex, Int32 endIndex)
   at Telerik.Windows.Controls.GridView.GridViewVirtualizingPanel.FlatLayoutStrategy.RealizeRows(Int32 startIndex, Int32 endIndex, Double& verticalOffset, HashSet`1& realizedRows)
   at Telerik.Windows.Controls.GridView.GridViewVirtualizingPanel.FlatLayoutStrategy.MeasureOverride(Size availableSize)
   at Telerik.Windows.Controls.GridView.GridViewVirtualizingPanel.MeasureOverride(Size availableSize)

NullReferenceException
   at Telerik.Windows.Data.QueryableCollectionView.InternalGetItemAt(Int32 index) in Telerik.Windows.Data\QueryableCollectionView.cs:line 3081
   at Telerik.Windows.Controls.GridView.Rows.GetRowItemsAtRange(Int32 startIndex, Int32 endIndex)
   at Telerik.Windows.Controls.GridView.GridViewVirtualizingPanel.FlatLayoutStrategy.RealizeRows(Int32 startIndex, Int32 endIndex, Double& verticalOffset, HashSet`1& realizedRows)
   at Telerik.Windows.Controls.GridView.GridViewVirtualizingPanel.FlatLayoutStrategy.MeasureOverride(Size availableSize)
   at Telerik.Windows.Controls.GridView.GridViewVirtualizingPanel.MeasureOverride(Size availableSize)

 Dispatcher.BeginInvoke(new Action(() =>
 {
    // add/remove items from the source collection
 }));