private void AddAppointment_OnClick(object sender, RoutedEventArgs e)
        {
            var selectedAppointment = this.AvailableAppointments.SelectedItem as AvailableAppointmentRowDto;
            var selectedSpecialist  = this.Specialists.SelectedIndex;


            if (selectedSpecialist < 0 || selectedAppointment == null)
            {
                return;
            }

            var specialist = this._currentSpecialists[selectedSpecialist];

            var appointment = new Appointment
            {
                Id           = Guid.NewGuid(),
                StartDate    = (DateTime)selectedAppointment.StartDate,
                EndDate      = (DateTime)selectedAppointment.EndDate,
                SpecialistId = specialist.Id
            };

            var appointmentService = new AppointmentsService();

            appointmentService.AddNewAppointment(appointment);
            MessageBox.Show("Dodano nową wizytę");
        }