@using System.ComponentModel.DataAnnotations
@using System.Collections.ObjectModel

@( theModel.Id == null ? "field is null" : theModel.Id.ToString() )

<EditForm Model="@theModel" OnValidSubmit="@Submit">
    <DataAnnotationsValidator />

    <TelerikComboBox @bind-Value="@theModel.Id" Data="@Tiers" ValueField="Id" TextField="Nom" TItem="TiersViewModel" TValue="long?" />

    <TelerikButton OnClick="@ShowTiersAddView" Icon="@IconName.Plus" ButtonType="@ButtonType.Button"></TelerikButton>

    <TelerikButton ButtonType="@ButtonType.Submit">SUBMIT</TelerikButton>
</EditForm>

@code {
    TheValidationModel theModel { get; set; }
    //private IList<TiersViewModel> Tiers { get; set; }
    private ObservableCollection<TiersViewModel> Tiers { get; set; }

    private async Task ShowTiersAddView() // note that the type of the method is "async Task" and not "async void"
    {
        Tiers.Clear();
        Tiers = new ObservableCollection<TiersViewModel>(GetTiers());

        await Task.Delay(20); // workaround

        theModel.Id = 2;

        await InvokeAsync(() => this.StateHasChanged());
    }

    protected override void OnInitialized()
    {
        theModel = new TheValidationModel();
        Tiers = new ObservableCollection<TiersViewModel>(GetTiers());
    }

    private IList<TiersViewModel> GetTiers()
    {
        return Enumerable.Range(1, 15).Select(x => new TiersViewModel { Id = x, Nom = $"Nom {x}" }).ToList();
    }

    private void Submit()
    {
        Console.WriteLine("valid form submit");
    }

    public class TiersViewModel
    {
        public long? Id { get; set; }
        public string Nom { get; set; }
    }

    public class TheValidationModel
    {
        [Required] public long? Id { get; set; }
    }
}