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)

<TelerikTreeView @ref="@TreeViewRef"
                 Data="@FlatData"
                 @bind-ExpandedItems="@ExpandedItems"
                 SelectionMode="@TreeViewSelectionMode.Multiple"
                 SelectedItems="@SelectedItems"
                 SelectedItemsChanged="@((IEnumerable<object> items) => SelectedItemsHandler(items))" />

<TelerikButton OnClick="@ExpandAll">Expand All</TelerikButton>
<TelerikButton OnClick="@CollapseAll">Collapse All</TelerikButton>

@code {
    public TelerikTreeView TreeViewRef { get; set; }
    public IEnumerable<TreeItem> FlatData { get; set; }
    public IEnumerable<object> SelectedItems { get; set; } = new List<object>();
    public IEnumerable<object> ExpandedItems { get; set; } = new List<object>();

    async Task ExpandAll()
    {
        ExpandedItems = FlatData.Where(x => x.HasChildren == true);

        await Task.Delay(1);
        TreeViewRef.Rebind();
    }

    void CollapseAll()
    {
        ExpandedItems = new List<object>();
        SelectedItems = new List<object>();
    }

    void SelectedItemsHandler(IEnumerable<object> items)
    {
        SelectedItems = items;
    }
    protected override async void OnInitialized()
    {
        FlatData = LoadFlat();
    }

    int TreeLevels { get; set; } = 3;
    int ItemsPerLevel { get; set; } = 3;
    int IdCounter { get; set; } = 1;
    List<TreeItem> LoadFlat()
    {
        List<TreeItem> items = new List<TreeItem>();
        PopulateTreeItems(items, null, 1);
        return items;
    }
    void PopulateTreeItems(List<TreeItem> items, int? parentId, int level)
    {
        for (int i = 1; i <= ItemsPerLevel; i++)
        {
            var itemId = IdCounter++;
            items.Add(new TreeItem()
            {
                Id = itemId,
                Text = $"Level {level} Item {i} Id {itemId}",
                ParentId = parentId,
                HasChildren = level < TreeLevels
            });
            if (level < TreeLevels)
            {
                PopulateTreeItems(items, itemId, level + 1);
            }
        }
    }

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

crit: Microsoft.AspNetCore.Components.WebAssembly.Rendering.WebAssemblyRenderer[100]
      Unhandled exception rendering component: Unknown edit type: 0
      Error: Unknown edit type: 0
          at e.applyEdits (https://localhost:44363/_framework/blazor.webassembly.js:1:15008)
          at e.updateComponent (https://localhost:44363/_framework/blazor.webassembly.js:1:12880)
          at Object.t.renderBatch (https://localhost:44363/_framework/blazor.webassembly.js:1:1704)
          at Object.window.Blazor._internal.renderBatch (https://localhost:44363/_framework/blazor.webassembly.js:1:34784)
          at _mono_wasm_invoke_js_unmarshalled (https://localhost:44363/_framework/wasm/dotnet.3.2.0.js:1:172099)
          at wasm_invoke_iiiiii (<anonymous>:wasm-function[3160]:0x9b33d)
          at icall_trampoline_dispatch (<anonymous>:wasm-function[5777]:0xfe711)
          at mono_wasm_interp_to_native_trampoline (<anonymous>:wasm-function[4607]:0xca81d)
          at ves_pinvoke_method (<anonymous>:wasm-function[3209]:0x9cd40)
          at interp_exec_method (<anonymous>:wasm-function[1120]:0x2598d)
Microsoft.JSInterop.JSException: Unknown edit type: 0
Error: Unknown edit type: 0
    at e.applyEdits (https://localhost:44363/_framework/blazor.webassembly.js:1:15008)
    at e.updateComponent (https://localhost:44363/_framework/blazor.webassembly.js:1:12880)
    at Object.t.renderBatch (https://localhost:44363/_framework/blazor.webassembly.js:1:1704)
    at Object.window.Blazor._internal.renderBatch (https://localhost:44363/_framework/blazor.webassembly.js:1:34784)
    at _mono_wasm_invoke_js_unmarshalled (https://localhost:44363/_framework/wasm/dotnet.3.2.0.js:1:172099)
    at wasm_invoke_iiiiii (<anonymous>:wasm-function[3160]:0x9b33d)
    at icall_trampoline_dispatch (<anonymous>:wasm-function[5777]:0xfe711)
    at mono_wasm_interp_to_native_trampoline (<anonymous>:wasm-function[4607]:0xca81d)
    at ves_pinvoke_method (<anonymous>:wasm-function[3209]:0x9cd40)
    at interp_exec_method (<anonymous>:wasm-function[1120]:0x2598d)
  at Microsoft.JSInterop.WebAssembly.WebAssemblyJSRuntime.InvokeUnmarshalled[T0,T1,T2,TResult] (System.String identifier, T0 arg0, T1 arg1, T2 arg2) <0x3ae01e8 + 0x00046> in <filename unknown>:0 
  at Microsoft.JSInterop.WebAssembly.WebAssemblyJSRuntime.InvokeUnmarshalled[T0,T1,TResult] (System.String identifier, T0 arg0, T1 arg1) <0x3ae0108 + 0x00014> in <filename unknown>:0 
  at Microsoft.AspNetCore.Components.WebAssembly.Rendering.WebAssemblyRenderer.UpdateDisplayAsync (Microsoft.AspNetCore.Components.RenderTree.RenderBatch& batch) <0x3ae0010 + 0x0001e> in <filename unknown>:0 
  at Microsoft.AspNetCore.Components.RenderTree.Renderer.ProcessRenderQueue () <0x387e448 + 0x000f2> in <filename unknown>:0

<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;
    }
}