private async void getAvaliableCars(object sender, EventArgs e)
 {
     cars.Clear();
     carTable.Rows.Clear();
     if (reservation.carType != 0 && reservation.dateBegin != null && reservation.dateEnd != null && reservation.dateBegin != "0000-00-00" && reservation.dateEnd != "0000-00-00")
     {
         await DatabaseService.getAvaliableCars(reservation).ContinueWith((task) =>
         {
             cars = task.Result;
             foreach (Car car in cars)
             {
                 if (carTable.InvokeRequired)
                 {
                     carTable.Invoke(new Action(() => addCarTableRow(car)));
                 }
                 else
                 {
                     addCarTableRow(car);
                 }
             }
             if (carTable.InvokeRequired)
             {
                 carTable.Invoke(new Action(() => carTable.Show()));
             }
             else
             {
                 carTable.Show();
             }
         });
     }
     else
     {
         MessageBox.Show("Uzupełnij wszystkie pola.");
     }
 }