System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
 ---> System.NotImplementedException: This is a DynamicProxy2 error: The interceptor attempted to 'Proceed' for method 'Void set_LazyLoader(Microsoft.EntityFrameworkCore.Infrastructure.ILazyLoader)' which has no target. 

            services.AddDbContext<ApplicationDbContext>(options =>
            options
            .UseLazyLoadingProxies() // this causes the problem
            .UseSqlServer(Configuration.GetConnectionString("DefaultConnection")), ServiceLifetime.Transient);

@using Telerik.DataSource

<p>Group by a third column, so that it should come last in the Group Panel:</p>

<TelerikGrid @ref="@GridRef"
             Data="@GridData"
             Pageable="true"
             Sortable="true"
             Groupable="true"
             FilterMode="GridFilterMode.FilterRow"
             OnStateInit="@( (GridStateEventArgs<Employee> args) => OnGridStateInit(args) )"
             OnStateChanged="@( (GridStateEventArgs<Employee> args) => OnGridStateChanged(args) )">
    <GridColumns>
        <GridColumn Field="@nameof(Employee.Name)" />
        <GridColumn Field="@nameof(Employee.Team)" />
        <GridColumn Field="@nameof(Employee.Salary)" />
        <GridColumn Field="@nameof(Employee.OnVacation)" />
    </GridColumns>
</TelerikGrid>

@code {
    private TelerikGrid<Employee>? GridRef { get; set; }

    private List<Employee> GridData { get; set; } = new();

    private void OnGridStateInit(GridStateEventArgs<Employee> args)
    {
        args.GridState.GroupDescriptors = new List<GroupDescriptor>();

        args.GridState.GroupDescriptors.Add(new GroupDescriptor()
        {
            Member = nameof(Employee.Team),
            MemberType = typeof(string)
        });

        args.GridState.GroupDescriptors.Add(new GroupDescriptor()
        {
            Member = nameof(Employee.OnVacation),
            MemberType = typeof(bool)
        });
    }

    private async Task OnGridStateChanged(GridStateEventArgs<Employee> args)
    {
        if (args.PropertyName == "GroupDescriptors" && args.GridState.GroupDescriptors.Count > 2 && GridRef != null)
        {
            var secondGroupDescriptor = args.GridState.GroupDescriptors.ElementAt(1);

            args.GridState.GroupDescriptors.Remove(secondGroupDescriptor);
            args.GridState.GroupDescriptors.Add(secondGroupDescriptor);

            await GridRef.SetStateAsync(args.GridState);
        }
    }

    protected override void OnInitialized()
    {
        var rnd = new Random();

        for (int i = 1; i <= 20; i++)
        {
            GridData.Add(new Employee()
            {
                Id = i,
                Name = "Name " + i,
                Team = "Team " + (i % 4 + 1),
                Salary = (decimal)rnd.Next(1000, 3000),
                OnVacation = i % 3 == 0
            });
        }
    }

    public class Employee
    {
        public int Id { get; set; }
        public string Name { get; set; } = string.Empty;
        public string Team { get; set; } = string.Empty;
        public decimal Salary { get; set; }
        public bool OnVacation { get; set; }
    }
}

@using Telerik.DataSource
@using Telerik.DataSource.Extensions

Scroll through the groups or expand them to load their data on demand

<TelerikGrid TItem="@object"
             LoadGroupsOnDemand="true"
             Groupable="true"
             OnStateInit="@((GridStateEventArgs<object> args) => OnStateInitHandler(args))"
             OnRead="@ReadItems"
             ScrollMode="@GridScrollMode.Virtual" PageSize="20" RowHeight="60"
             Navigable="true" Sortable="true" FilterMode="@GridFilterMode.FilterRow" Height="600px">
    <GridColumns>
        <GridColumn Field="@nameof(Employee.Name)" FieldType="@typeof(string)" Groupable="false" />
        <GridColumn Field="@nameof(Employee.Team)" FieldType="@typeof(string)" Title="Team" />
        <GridColumn Field="@nameof(Employee.Salary)" FieldType="@typeof(decimal)" Groupable="false" />
        <GridColumn Field="@nameof(Employee.IsOnLeave)" FieldType="@typeof(bool)" Title="On Vacation" />
    </GridColumns>
</TelerikGrid>

@code {
    List<object> GridData { get; set; }

    protected async Task ReadItems(GridReadEventArgs args)
    {
        // sample data retrieval, see comments in the service mimic class below
        DataEnvelope<Employee> result = await MyService.GetData(args.Request);

        if (args.Request.Groups.Count > 0)
        {
            args.Data = result.GroupedData.Cast<object>().ToList();
        }
        else
        {
            args.Data = result.CurrentPageData.Cast<object>().ToList();
        }

        args.Total = result.TotalItemCount;
    }

    void OnStateInitHandler(GridStateEventArgs<object> args)
    {
        // set initial grouping
        GridState<object> desiredState = new GridState<object>()
        {
            GroupDescriptors = new List<GroupDescriptor>()
            {
                new GroupDescriptor()
                {
                    Member = "Team",
                    MemberType = typeof(string)
                },
                new GroupDescriptor()
                {
                    Member = "IsOnLeave",
                    MemberType = typeof(bool)
                }
            }
        };

        args.GridState = desiredState;
    }

    public class Employee
    {
        public int EmployeeId { get; set; }
        public string Name { get; set; }
        public string? Team { get; set; }
        public bool IsOnLeave { get; set; }
        public decimal Salary { get; set; }
    }

    public class DataEnvelope<T>
    {
        public List<AggregateFunctionsGroup> GroupedData { get; set; }
        public List<T> CurrentPageData { get; set; }
        public int TotalItemCount { get; set; }
    }

    public static class MyService
    {
        private static List<Employee> SourceData { get; set; }
        public static async Task<DataEnvelope<Employee>> GetData(DataSourceRequest request)
        {
            if (SourceData == null)
            {
                SourceData = new List<Employee>();
                var rand = new Random();
                for (int i = 1; i <= 3; i++)
                {
                    SourceData.Add(new Employee()
                    {
                        EmployeeId = i,
                        Name = "Employee " + i.ToString(),
                        Team = "Team " + i % 100,
                        IsOnLeave = i % 3 == 0,
                        Salary = rand.Next(1000, 5000)
                    });
                }
                SourceData.Add(new Employee()
                    {
                        EmployeeId = 3,
                        Name = "Employee " + 3.ToString(),
                        Team = null,
                        IsOnLeave = 3 % 3 == 0,
                        Salary = rand.Next(1000, 5000)
                    });
            }

            await Task.Delay(500);// deliberate delay to showcase async operations, remove in a real app

            // retrieve data as needed, you can find more examples and runnable projects here
            // https://github.com/telerik/blazor-ui/tree/master/grid/datasourcerequest-on-server
            var datasourceResult = SourceData.ToDataSourceResult(request);

            DataEnvelope<Employee> dataToReturn;

            if (request.Groups.Count > 0)
            {
                dataToReturn = new DataEnvelope<Employee>
                {
                    GroupedData = datasourceResult.Data.Cast<AggregateFunctionsGroup>().ToList(),
                    TotalItemCount = datasourceResult.Total
                };
            }
            else
            {
                dataToReturn = new DataEnvelope<Employee>
                {
                    CurrentPageData = datasourceResult.Data.Cast<Employee>().ToList(),
                    TotalItemCount = datasourceResult.Total
                };
            }

            return await Task.FromResult(dataToReturn);
        }
    }
}