System.NullReferenceException: Object reference not set to an instance of an object.
   at Telerik.Blazor.Components.TelerikGridBase`1.FocusPagerAsync()
   at Telerik.Blazor.Components.TelerikGridBase`1.FocusCellAsync(GridNavigationCommandEventArgs args)
   at Telerik.Blazor.Components.TelerikGridBase`1.FocusAdjacentCellAsync(GridNavigationCommandEventArgs args, Int32 rowIndexOffset, Int32 columnIndexOffset)
   at Telerik.Blazor.Components.TelerikGridBase`1.FocusBottomCellAsync(GridNavigationCommandEventArgs args)
   at Telerik.Blazor.Components.TelerikGridBase`1.ExecuteNavigationCommandAsync(GridNavigationCommandEventArgs args)
   at Telerik.Blazor.Components.TelerikGridBase`1.ExecuteCommand(Object args)
   at Telerik.Blazor.Components.Grid.GridRowBase`1.OnExecuteCommand(GridCommandEventArgs commandArgs)
   at Telerik.Blazor.Components.Grid.GridDataCellBase`1.ExecuteCommandAsync(GridCommandEventArgs args)
   at Telerik.Blazor.Components.Grid.GridNavigableCellBase`1.ExecuteNavigationCommandAsync(String commandName, Int32 rowIndexOffset, Int32 columnIndexOffset, KeyboardEventArgs args)
   at Telerik.Blazor.Components.Grid.GridNavigableCellBase`1.ProcessKeyDown(KeyboardEventArgs args)
   at Telerik.Blazor.Components.Grid.GridContentCell`1.ProcessKeyDown(KeyboardEventArgs args)
   at Telerik.Blazor.Components.Grid.GridNavigableCellBase`1.OnKeyDown(KeyboardEventArgs args)
   at Microsoft.AspNetCore.Components.ComponentBase.CallStateHasChangedOnAsyncCompletion(Task task)
   at Microsoft.AspNetCore.Components.RenderTree.Renderer.GetErrorHandledTask(Task taskToHandle)

Error: System.ObjectDisposedException: Cannot access a disposed object.

Object name: 'DotNetObjectReference`1'.

   at Microsoft.JSInterop.DotNetObjectReference`1.ThrowIfDisposed()

   at Microsoft.JSInterop.JSRuntime.TrackObjectReference[TValue](DotNetObjectReference`1 dotNetObjectReference)

   at Microsoft.JSInterop.Infrastructure.DotNetObjectReferenceJsonConverter`1.Write(Utf8JsonWriter writer, DotNetObjectReference`1 value, JsonSerializerOptions options)

   at System.Text.Json.JsonSerializer.WriteDictionary[TProperty](JsonConverter`1 converter, JsonSerializerOptions options, WriteStackFrame& current, Utf8JsonWriter writer)

   at System.Text.Json.JsonPropertyInfoNotNullable`4.OnWriteDictionary(WriteStackFrame& current, Utf8JsonWriter writer)

   at System.Text.Json.JsonPropertyInfo.WriteDictionary(WriteStack& state, Utf8JsonWriter writer)

   at System.Text.Json.JsonSerializer.HandleDictionary(JsonClassInfo elementClassInfo, JsonSerializerOptions options, Utf8JsonWriter writer, WriteStack& state)

   at System.Text.Json.JsonSerializer.Write(Utf8JsonWriter writer, Int32 originalWriterDepth, Int32 flushThreshold, JsonSerializerOptions options, WriteStack& state)

   at System.Text.Json.JsonSerializer.WriteCore(Utf8JsonWriter writer, Object value, Type type, JsonSerializerOptions options)

   at System.Text.Json.JsonSerializer.WriteCore(PooledByteBufferWriter output, Object value, Type type, JsonSerializerOptions options)

   at System.Text.Json.JsonSerializer.WriteCoreString(Object value, Type type, JsonSerializerOptions options)

   at Microsoft.JSInterop.JSRuntime.InvokeAsync[TValue](String identifier, CancellationToken cancellationToken, Object[] args)

   at Microsoft.JSInterop.JSRuntime.InvokeWithDefaultCancellation[T](String identifier, Object[] args)

   at Telerik.Blazor.Components.Grid.GridHeaderRowBase`1.InitColumnReorderable()

   at Telerik.Blazor.Components.Grid.GridHeaderRowBase`1.OnAfterRenderAsync(Boolean firstRender)

   at Microsoft.AspNetCore.Components.RenderTree.Renderer.GetErrorHandledTask(Task taskToHandle)

<TelerikGrid Data="salesTeamMembers" PageSize="4" Pageable="true" @bind-SelectedItems="@SelectedItemsMainGrid" SelectionMode="@GridSelectionMode.Single">
    <DetailTemplate>
        @{
            var employee = context as MainModel;
            <TelerikGrid Data="employee.Orders" Pageable="true" PageSize="7" SelectionMode="@GridSelectionMode.None">
                <GridColumns>
                    <GridColumn Field="OrderId"></GridColumn>
                    <GridColumn Field="DealSize"></GridColumn>
                </GridColumns>
            </TelerikGrid>
        }
    </DetailTemplate>
    <GridColumns>
        <GridColumn Field="Id"></GridColumn>
        <GridColumn Field="Name"></GridColumn>
    </GridColumns>
</TelerikGrid>

@foreach (var item in SelectedItemsMainGrid)
{
    <div>@item.Name</div>
}

@code {
    List<MainModel> salesTeamMembers { get; set; }
    public IEnumerable<MainModel> SelectedItemsMainGrid { get; set; } = Enumerable.Empty<MainModel>();

    protected override void OnInitialized()
    {
        salesTeamMembers = GenerateData();
    }

    private List<MainModel> GenerateData()
    {
        List<MainModel> data = new List<MainModel>();
        for (int i = 1; i < 16; i++)
        {
            MainModel mdl = new MainModel { Id = i, Name = $"Name {i}" };
            mdl.Orders = Enumerable.Range(1, 15).Select(x => new DetailsModel { OrderId = x, DealSize = x ^ i }).ToList();
            data.Add(mdl);
        }
        return data;
    }

    public class MainModel
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public List<DetailsModel> Orders { get; set; }
    }

    public class DetailsModel
    {
        public int OrderId { get; set; }
        public double DealSize { get; set; }
    }
}