@page "/"
<TelerikTreeView Data="@FlatData"
                 CheckBoxMode="@TreeViewCheckBoxMode.Multiple"
                 CheckParents="@true"
                 CheckChildren="@true"
                 CheckOnClick="@false"
                 @bind-CheckedItems="@SelectedItems">
</TelerikTreeView>

@code {
    List<TreeItem> FlatData { get; set; }
    IEnumerable<object> SelectedItems { get; set; } = new List<object>();
    protected override async Task OnInitializedAsync()
    {
        await GenerateData();
        await SelectDefault();
    }

    async Task SelectDefault()
    {
        await Task.Delay(100);
        SelectedItems = FlatData.Where(data => data.Id == 2);
    }
#pragma warning disable
    async Task GenerateData()
    {
        FlatData = new List<TreeItem>();

        FlatData.Add(new TreeItem()
            {
                Id = 1,
                Text = "Project",
                ParentId = null,
                HasChildren = true,
                Icon = "folder",
                Expanded = true
            });

        FlatData.Add(new TreeItem()
            {
                Id = 2,
                Text = "Design",
                ParentId = 1,
                HasChildren = true,
                Icon = "brush",
                Expanded = true
            });
        FlatData.Add(new TreeItem()
            {
                Id = 3,
                Text = "Implementation",
                ParentId = 1,
                HasChildren = true,
                Icon = "folder",
                Expanded = true
            });
    }

    public class TreeItem
    {
        public int Id { get; set; }
        public string Text { get; set; }
        public int? ParentId { get; set; }
        public bool HasChildren { get; set; }
        public string Icon { get; set; }
        public bool Expanded { get; set; }
    }
}

@using System.Collections.ObjectModel

<TelerikButton OnClick="@AddItem">Add Item</TelerikButton>

<TelerikButton OnClick="@RemoveItem">Remove Item</TelerikButton>

<p>
    TreeViewCheckedItems Count: @TreeViewCheckedItems.Count()
    <br />
    <label><TelerikCheckBox @bind-Value="@ApplyWorkaround" /> Apply Workaround</label>
</p>

<TelerikTreeView Data="@TreeViewData"
                 CheckBoxMode="@TreeViewCheckBoxMode.Multiple"
                 @bind-CheckedItems="@TreeViewCheckedItems" />

@code {
    private IEnumerable<object> TreeViewCheckedItems { get; set; } = new List<TreeItem>();
    private ObservableCollection<object> TreeViewData { get; set; } = new ObservableCollection<object>();

    private int LastId { get; set; }
    private bool ApplyWorkaround { get; set; }

    private void AddItem()
    {
        TreeViewData.Add(
            new TreeItem
            {
                Id = ++LastId,
                Text = $"Item {LastId}",
            });

        if (ApplyWorkaround)
        {
            TreeViewCheckedItems = new List<object>(TreeViewCheckedItems);
        }

    }

    private void RemoveItem()
    {
        if (TreeViewData.Count > 1)
        {
            TreeViewData.Remove(TreeViewData.Last());

            if (ApplyWorkaround)
            {
                TreeViewCheckedItems = new List<object>(TreeViewCheckedItems);
            }
        }
    }

    protected override void OnInitialized()
    {
        TreeViewData = new ObservableCollection<object>() {
            new TreeItem()
            {
                Id = ++LastId,
                Text = $"Item {LastId}"
            }
        };

        TreeViewCheckedItems = new List<object>() { TreeViewData.First() };
    }
    public class TreeItem
    {
        public int Id { get; set; }
        public int? ParentId { get; set; }
        public string Text { get; set; } = string.Empty;
        public bool HasChildren { get; set; }
    }
}

<button @onclick="@SwitchDataSource">switch to other data source</button>

<TelerikTreeView Data="@FlatData">
    <TreeViewBindings>
        <TreeViewBinding IdField="Id" ParentIdField="ParentIdValue" ExpandedField="Expanded" TextField="Text" 
                         HasChildrenField="HasChildren" IconField="Icon">
            <ItemTemplate>
                @{
                    TreeItem currProduct = context as TreeItem;
                    @(currProduct.Text)
                }
            </ItemTemplate>
            </TreeViewBinding>
    </TreeViewBindings>
</TelerikTreeView>

@code {
    public class TreeItem
    {
        public int Id { get; set; }
        public string Text { get; set; }
        public int? ParentIdValue { get; set; }
        public bool HasChildren { get; set; }
        public string Icon { get; set; }
        public bool Expanded { get; set; }
    }

    async void SwitchDataSource()
    {
        //workaround - remove it to see the actual error
        foreach (TreeItem item in FlatData)
        {
            item.Expanded = false;
        }
        StateHasChanged();
        await Task.Delay(300);//awaits the animation that will hide the nodes we just collapsed so their elements get properly disposed
        
        //change data
        LoadSecondDataSource();
        //update UI
        StateHasChanged();
    }

    public IEnumerable<TreeItem> FlatData { get; set; }

    protected override void OnInitialized()
    {
        LoadFlatData();
    }

    private void LoadFlatData()
    {
        List<TreeItem> items = new List<TreeItem>();

        items.Add(new TreeItem()
        {
            Id = 1,
            Text = "Project",
            ParentIdValue = null,
            HasChildren = true,
            Icon = "folder",
            Expanded = true
        });

        items.Add(new TreeItem()
        {
            Id = 2,
            Text = "Design",
            ParentIdValue = 1,
            HasChildren = true,
            Icon = "brush",
            Expanded = true
        });
        items.Add(new TreeItem()
        {
            Id = 3,
            Text = "Implementation",
            ParentIdValue = 1,
            HasChildren = true,
            Icon = "folder",
            Expanded = true
        });

        items.Add(new TreeItem()
        {
            Id = 4,
            Text = "site.psd",
            ParentIdValue = 2,
            HasChildren = false,
            Icon = "psd",
            Expanded = true
        });
        items.Add(new TreeItem()
        {
            Id = 5,
            Text = "index.js",
            ParentIdValue = 3,
            HasChildren = false,
            Icon = "js"
        });
        items.Add(new TreeItem()
        {
            Id = 6,
            Text = "index.html",
            ParentIdValue = 3,
            HasChildren = false,
            Icon = "html"
        });
        items.Add(new TreeItem()
        {
            Id = 7,
            Text = "styles.css",
            ParentIdValue = 3,
            HasChildren = false,
            Icon = "css"
        });

        FlatData = items;
    }

    void LoadSecondDataSource()
    {
        List<TreeItem> items = new List<TreeItem>();

        items.Add(new TreeItem()
        {
            Id = 1,
            Text = "1",
            ParentIdValue = null,
            HasChildren = true
        });

        items.Add(new TreeItem()
        {
            Id = 2,
            Text = "1 1",
            ParentIdValue = 1,
            HasChildren = true
        });


        //if you add this, there is no error because the levels match
        //if the new data source does not have the same number of (maybe expanded) levels
        //you will get an error while disposing those levels
        //items.Add(new TreeItem()
        //{
        //    Id = 3,
        //    Text = "1 1 1",
        //    ParentIdValue = 2
        //});



        FlatData = items;
    }
}

System.NullReferenceException: Object reference not set to an instance of an object.
   at Telerik.Blazor.Components.Common.Trees.TelerikTreeDataSource.GetTreeItems(IEnumerable`1 data, Int32 level, String parentIndex)