public partial class App : Application
    {
        protected override void OnStartup(StartupEventArgs e)
        {
            base.OnStartup(e);

            var connectionViewModel = new ConnectionDBViewModel();
            var authorizationForm = new AuthorizationForm(connectionViewModel);

            EventHandler onSuccessfulСonnection = null;

            onSuccessfulСonnection = async (obj, ev) =>
            {
                connectionViewModel.SuccessfulСonnection -= onSuccessfulСonnection;

                Current.MainWindow = new MainWindow(await SpreadsheetsViewModel.NewAsync(connectionViewModel));
                authorizationForm.Close();
                Current.MainWindow.Show();
            };

            connectionViewModel.SuccessfulСonnection += onSuccessfulСonnection;

            authorizationForm.Show();
        }
    }



public event EventHandler SuccessfulСonnection = null;

public ICommand LogInCommand
        {
            get
            {
                return Get(() => LogInCommand, new RelayCommand((sender) =>
                {
                    if (IsValidOnClick())
                    {
                        if ((sender as Storyboard) != null)
                        {
                            EventHandler onCompleted = null;

                            onCompleted = async (obj, e) =>
                            {
                                (sender as Storyboard).Completed -= onCompleted;

                                try
                                {
                                    using (var sqlConnection = LinqHelpers.NewConnection(SrvName, DbName, UsrName, PasName, SSPI))
                                    {
                                        await sqlConnection.OpenAsync();
                                        SchemaDatabase = await LinqHelpers.GetSchemaAsync(sqlConnection, "Databases", new string[] { DbName });
                                    }

                                    SuccessfulСonnection?.Invoke(this, EventArgs.Empty);
                                }
                                catch (Exception ex)
                                {
                                    errorCallback.ErrorInfo = ex.Message;
                                }
                            };

                            (sender as Storyboard).Completed += onCompleted;
                            (sender as Storyboard).Begin();
                        }
                    }
                }));
            }
        }