Error: System.InvalidOperationException: No generic method 'Sum' on type 'System.Linq.Enumerable' is compatible with the supplied type arguments and arguments. No type arguments should be provided if the method is non-generic. 
   at System.Linq.Expressions.Expression.FindMethod(Type type, String methodName, Type[] typeArgs, Expression[] args, BindingFlags flags)
   at System.Linq.Expressions.Expression.Call(Type type, String methodName, Type[] typeArguments, Expression[] arguments)
   at Telerik.DataSource.Expressions.EnumerableSelectorAggregateFunctionExpressionBuilder.CreateMethodCallExpression(LambdaExpression memberSelectorExpression)
   at Telerik.DataSource.Expressions.EnumerableSelectorAggregateFunctionExpressionBuilder.CreateAggregateExpression()
   at Telerik.DataSource.EnumerableSelectorAggregateFunction.CreateAggregateExpression(Expression enumerableExpression, Boolean liftMemberAccessToNull)
   at Telerik.DataSource.Expressions.GroupDescriptorExpressionBuilder.<ProjectionPropertyValueExpressions>b__39_0(AggregateFunction f)
   at System.Linq.Enumerable.SelectIListIterator`2.ToList()
   at Telerik.DataSource.Expressions.GroupDescriptorExpressionBuilder.CreateProjectionInitExpression()
   at Telerik.DataSource.Expressions.GroupDescriptorExpressionBuilder.CreateAggregateFunctionsProjectionMemberBinding()
   at Telerik.DataSource.Expressions.QueryableAggregatesExpressionBuilder.CreateMemberBindings()+MoveNext()
   at System.Collections.Generic.LargeArrayBuilder`1.AddRange(IEnumerable`1 items)
   at System.Collections.Generic.EnumerableHelpers.ToArray[T](IEnumerable`1 source)
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at System.Dynamic.Utils.CollectionExtensions.ToReadOnly[T](IEnumerable`1 enumerable)
   at System.Linq.Expressions.Expression.MemberInit(NewExpression newExpression, IEnumerable`1 bindings)
   at Telerik.DataSource.Expressions.GroupDescriptorExpressionBuilder.CreateSelectBodyExpression()
   at Telerik.DataSource.Expressions.GroupDescriptorExpressionBuilder.CreateSelectExpression()
   at Telerik.DataSource.Expressions.GroupDescriptorExpressionBuilderBase.CreateQuery()
   at Telerik.DataSource.Extensions.QueryableExtensions.Aggregate(IQueryable source, IEnumerable`1 aggregateFunctions)
   at Telerik.DataSource.Extensions.QueryableExtensions.CreateDataSourceResult[TModel,TResult](IQueryable queryable, DataSourceRequest request, Func`2 selector)
   at Telerik.DataSource.Extensions.QueryableExtensions.ToDataSourceResult(IQueryable queryable, DataSourceRequest request)
   at Telerik.DataSource.Extensions.QueryableExtensions.ToDataSourceResult(IEnumerable enumerable, DataSourceRequest request)
   at Telerik.Blazor.Data.TelerikDataSourceBase.ProcessData(IEnumerable data)
   at Telerik.Blazor.Components.Common.DataBoundComponent`1.ProcessDataInternal()
   at Telerik.Blazor.Components.Common.DataBoundComponent`1.ProcessDataAsync()
   at Telerik.Blazor.Components.TelerikGrid`1.DataBoundProcessData()
   at Telerik.Blazor.Components.TelerikGrid`1.ProcessDataAsync()
   at Telerik.Blazor.Components.TelerikGrid`1.ApplyFiltersAsync()
   at Telerik.Blazor.Components.TelerikGrid`1.OnFilterChange(FilterDescriptorBase filter)
   at Telerik.Blazor.Components.Common.Filters.TelerikFilterHeader`1.Filter(FilterDescriptor filterDescriptor)
   at Telerik.Blazor.Components.Common.Filters.TelerikFilterHeader`1.OnValueChanged(Object newValue)
   at System.Threading.Tasks.Task.<>c.<ThrowAsync>b__139_0(Object state)
   at Microsoft.AspNetCore.Components.Rendering.RendererSynchronizationContext.ExecuteSynchronously(TaskCompletionSource`1 completion, SendOrPostCallback d, Object state)
   at Microsoft.AspNetCore.Components.Rendering.RendererSynchronizationContext.<>c.<.cctor>b__23_0(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location where exception was thrown ---
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at Microsoft.AspNetCore.Components.Rendering.RendererSynchronizationContext.ExecuteBackground(WorkItem item)

[13:07:52 ERR] Unhandled exception in circuit 'hQsJqY3F4XbTbzQETSggJClg_e4YRyQkrKHFMHCXugM'.
System.Threading.Tasks.TaskCanceledException: A task was canceled.
   at Microsoft.JSInterop.JSRuntime.InvokeAsync[TValue](Int64 targetInstanceId, String identifier, Object[] args)
   at Microsoft.JSInterop.JSRuntimeExtensions.InvokeVoidAsync(IJSRuntime jsRuntime, String identifier, Object[] args)
   at Telerik.Blazor.Components.TelerikComboBox`2.Dispose()
   at System.Threading.Tasks.Task.<>c.<ThrowAsync>b__140_0(Object state)
   at Microsoft.AspNetCore.Components.Rendering.RendererSynchronizationContext.ExecuteSynchronously(TaskCompletionSource`1 completion, SendOrPostCallback d, Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location ---
   at Microsoft.AspNetCore.Components.Rendering.RendererSynchronizationContext.ExecuteBackground(WorkItem item)

<TelerikGrid Data="strategicLevelItems" SelectedItems="selectedStrategic" Width="100%" Height="500px"
                                 ScrollMode="GridScrollMode.Scrollable" SelectionMode="GridSelectionMode.Multiple"
                                 SelectedItemsChanged="@((IEnumerable<GetNavigationNodesModel> strategicItems) => OnStrategicSelectAsync(strategicItems))"
                                 FilterMode="GridFilterMode.FilterRow">
                        <GridColumns>
                            <GridCheckboxColumn SelectAll="true" Width="40px" OnCellRender="@GridHelpers.CenterAlign" />
                            <GridColumn Field="@(nameof(GetNavigationNodesModel.Name))" />
                        </GridColumns>
                    </TelerikGrid>

    private async Task OnStrategicSelectAsync(IEnumerable<GetNavigationNodesModel> selectedItems)
    {
        selectedStrategic = selectedItems;

        var state = tacticalGrid.GetState();

        var compositeFilter = new CompositeFilterDescriptor() { LogicalOperator = FilterCompositionLogicalOperator.Or };
        foreach (var item in selectedItems)
        {
            compositeFilter.FilterDescriptors.Add(new FilterDescriptor()
            {
                Member = "ParentId",
                Operator = FilterOperator.IsEqualTo,
                Value = item.Id
            });
        }

        state.FilterDescriptors.Clear();
        state.FilterDescriptors.Add(compositeFilter);

        await tacticalGrid.SetState(state);
    }

@inject IJSRuntime JsInterop

<TelerikGrid Data="@salesTeamMembers" OnRowClick="@OnRowClickHandler" @ref="@GridRef">
    <DetailTemplate>
        @{
            var employee = context as MainModel;
            <TelerikGrid Data="employee.Orders" Pageable="true" PageSize="5">
                <GridColumns>
                    <GridColumn Field="OrderId"></GridColumn>
                    <GridColumn Field="DealSize"></GridColumn>
                </GridColumns>
            </TelerikGrid>
        }
    </DetailTemplate>
    <GridColumns>
        <GridColumn Field="Id"></GridColumn>
        <GridColumn Field="Name"></GridColumn>
    </GridColumns>
</TelerikGrid>

@code {
    List<MainModel> salesTeamMembers { get; set; }
    TelerikGrid<MainModel> GridRef { get; set; }

    async Task OnRowClickHandler(GridRowClickEventArgs args) {
        // After adding this line, it now requires a double click when the InvokeAsync call uses "await"
         var width = JsInterop.InvokeAsync<int>("getWidth");


        var model = args.Item as MainModel;
        int index = salesTeamMembers.IndexOf(model);
        //todo: you may want to take paging into account for example, or use js interop to get the index of the row based on contents from it like id
        if (index > -1) {
            var state = GridRef.GetState();
            state.ExpandedRows = new List<int> { index };

            await GridRef.SetState(state);
        }

    }


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

    private List<MainModel> GenerateData() {
        List<MainModel> data = new List<MainModel>();
        for (int i = 0; i < 5; 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; }
    }
}

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)

Error: System.ArgumentNullException: Value cannot be null. (Parameter 'nullableType')

@using System.Collections.ObjectModel;

<div style="align-self:center;text-align:center" class="alert-danger">
    @errorMessages
</div>
<div style="align-self:center;text-align:center" class="alert-success">
    @successMessages
</div>
<TelerikGrid Data=@entitlementsExtended
             Pageable="true"
             Groupable="false"
             PageSize="@PageSize"
             OnUpdate="@UpdateHandler"
             Sortable="false"
             FilterMode="Telerik.Blazor.GridFilterMode.FilterMenu">
    <GridColumns>
        <GridColumn Field="@nameof(EntitlementValueDTOExtended.Description)" Editable="false" />
        <GridColumn Field=@nameof(EntitlementValueDTOExtended.Value)>
            <EditorTemplate>
                @{
                    CurrentlyEditedEntitlement = context as EntitlementValueDTOExtended;
                    <div>
                        <TelerikDropDownList Data="@entitlementOptions" @bind-Value="CurrentlyEditedEntitlement.Value" Width="60px" PopupHeight="auto"></TelerikDropDownList>
                    </div>
                }
            </EditorTemplate>
        </GridColumn>

        <GridCommandColumn Width="300px">
            <GridCommandButton Command="Save" Icon="save" ShowInEdit="true">Update</GridCommandButton>
            <GridCommandButton Command="Edit" Icon="edit">Edit</GridCommandButton>
            <GridCommandButton Command="Cancel" Icon="cancel" ShowInEdit="true">Cancel</GridCommandButton>
        </GridCommandColumn>
    </GridColumns>
</TelerikGrid>

@code {
    int PageSize = 10;
    public EntitlementValueDTOExtended CurrentlyEditedEntitlement { get; set; } = new EntitlementValueDTOExtended();

        //changing this to List<> works
    public ObservableCollection<EntitlementValueDTOExtended> entitlementsExtended { get; set; } = new ObservableCollection<EntitlementValueDTOExtended>();

    public static List<string> entitlementOptions = new List<string> { "I", "E" };

    public string errorMessages { get; set; } = "";
    public string successMessages { get; set; } = "";

    protected async override Task OnInitializedAsync()
    {
        IEnumerable<int> entitlements = Enumerable.Range(1, 50);

        //this is a simplified workaround for people needing nested models
        foreach (var entitlement in entitlements)
        {
            EntitlementValueDTOExtended entitlementExtended = new EntitlementValueDTOExtended();
            entitlementExtended.Id = Guid.NewGuid();
            entitlementExtended.Description = $"descr {entitlement}";
            entitlementExtended.Value = $"value {entitlement}";
            entitlementExtended.EntitlementTypeId = entitlement;

            entitlementsExtended.Add(entitlementExtended);
        }
        StateHasChanged();

        await base.OnInitializedAsync();
    }

    public async Task UpdateHandler(GridCommandEventArgs args)
    {
        EntitlementValueDTOExtended item = (EntitlementValueDTOExtended)args.Item;

        var matchingItem = entitlementsExtended.FirstOrDefault(c => c.Id == item.Id);
        if (matchingItem != null)
        {
            matchingItem.Description = item.Description;
            matchingItem.EntitlementTypeId = item.EntitlementTypeId;
            matchingItem.Value = item.Value;
        }

        successMessages = $"updated grid on {DateTime.Now}";
        StateHasChanged();

    }

    public class EntitlementValueDTOExtended
    {
        public Guid Id { get; set; }
        public string Description { get; set; } = "";
        public string Value { get; set; } = "";
        public int EntitlementTypeId { get; set; } = 0;
    }