I want to select the new node and set it in editing mode, but the ItemContainer is always null.
        I read all the forum posts and How To's but nothing works for me. The new item is selected and visible (it's parent is expanded).
        Also I tried the ExpandItemByPath doesn't do anything on my tree and GetItemByPath returns null.
      
        See the AddItem() method in code below:

Code behind:

public partial class MyTreeList
{
        public MyTreeList()
        {
            InitializeComponent();
            this.DataContext = GenerateList();
            _rootItem = list;
        }
        
        public static ItemVM GenerateList()
        {
            var root = new ItemVM() {Title = "Root"};
            var item1 = new ItemVM() { Title = "Item 1" };
            var item11 = new ItemVM() { Title = "Item 1-1" };
            var item12 = new ItemVM() { Title = "Item 1-2" };
            item1.References.Add(item11);
            item1.References.Add(item12);

            var item2 = new ItemVM() { Title = "Item 2" };
            var item21 = new ItemVM() { Title = "Item 2-1" };
            var item22 = new ItemVM() { Title = "Item 2-2" };
            item2.References.Add(item21);
            item2.References.Add(item22);

            root.References.Add(item1);
            root.References.Add(item2);

            //var collection = new ObservableCollection<ItemVM>() {root};                        
            return root;
        }    
        
        private void AddItem()
        {
            var node = _radTreeView.SelectedContainer;
            var newItem = new ItemVM(){Title = "xxx"};
            AddItem(node, newItem);
        }
        
        private void AddItem(RadTreeViewItem parentContainer, ItemVM newItem)
        {            
            _newItem = (ItemVM)newItem;
            
            if (parentContainer == null)
            {
                return;
            }
            else
            {            
                //Add new item to the list - works fine
                var item = parentContainer.Item as ItemVM;
                item.References.Add(newItem);
                parentContainer.IsExpanded = true;
                
                // Next want to set the new item into editable mode - nothing works
                // Element is selected and visible but container is null, programatic expanding doesn't do anything
                
                _rootItem.SelectedItem = newItem; - will select new item but doesn't help with the
                // returns null                
                var newContainer = _radTreeView.ContainerFromItemRecursive(newItem);
                //newContainer.IsInEditMode = true;
                
                newContainer = _radTreeView.ContainerFromItemRecursive(newItem);
                //still null
                newContainer = parentContainer.ItemContainerGenerator.ContainerFromItem(newItem);
                //still null
                
                var oldContainer = _radTreeView.ContainerFromItemRecursive(item);
                              
                // Different try (hardcoded path to try it out -- clicked add on "Item 1")              
                // Expanding doesn't do anything - item is not expanded
                string path = string.Format("Item 1|xxx");
                _radTreeView.ExpandItemByPath(path, "|");    
                var x = _radTreeView.GetItemByPath(path,"|");        //item is null        
            }
        }
}       
XAML:
<UserControl.Resources>
            <telerik:HierarchicalDataTemplate x:Key="Item" ItemsSource="{Binding References}">
                <StackPanel Orientation="Horizontal">
                    <TextBlock Text="{Binding Title}" />
                </StackPanel>
            </telerik:HierarchicalDataTemplate>    
        </UserControl.Resources>
        
<telerik:RadTreeView x:Name="_radTreeView" Margin="10"
                             ItemsIndent="30"
                             SelectionMode="Extended"
                             IsLineEnabled="False"                             
                             ItemsOptionListType="CheckList"
                             IsOptionElementsEnabled="True"
                             IsTriStateMode="True"
                             IsRootLinesEnabled="True"                                                           
                             IsExpandOnDblClickEnabled="false"
                             IsExpandOnSingleClickEnabled="False"
                             IsDragDropEnabled="True"
                             ItemTemplate="{StaticResource Item}"
                             IsEditable="True"
                             ItemsSource="{Binding Path=References}"
                             ItemClick="RadTreeView_ItemClick"
                             ItemDoubleClick="RadTreeView_ItemDoubleClick"
                             SelectedItem="{Binding SelectedItem, Mode=TwoWay}"
                             >
            <telerik:RadContextMenu.ContextMenu>
                <telerik:RadContextMenu x:Name="contextMenu" Opened="ContextMenuOpened"
                                        ItemClick="MenuItemClicked"
                                        >
                    <telerik:RadMenuItem Header="Add New Item" />
                    <telerik:RadMenuItem Header="Remove Item" />                    
                </telerik:RadContextMenu>
            </telerik:RadContextMenu.ContextMenu>
</telerik:RadTreeView>   

ViewModel

public class ItemVM : VMBase
    {
        private string _title;
        public string Title
        {
            get { return _title; }
            set
            {
                _title = value;
                NotifyPropertyChanged("Title");
            }
        }
        public IList<ItemVM> _references;
        public IList<ItemVM> References
        {
            get { return _references; }
            set
            {
                _references = value;
                NotifyPropertyChanged("References");
            }
        }
    }

I fixed it already. ExpandItemByPath had wrong path. After calling it with the right path it works.
Seems that it really has to be called no matter if the item is visible or not.