private void BtnAdd_Click(object sender, RoutedEventArgs e)
        {
            if (editMode)
            {
                ToggleEditMode(false);
            }

            if (addMode)
            {
                if (ValidateFields())
                {
                    fuelPurchases.Add(
                        vehicle,
                        decimal.Parse(tbCost.Text),
                        double.Parse(tbLitres.Text),
                        (DateTime)dpDate.SelectedDate
                        );

                    changes = true;
                    ToggleAddMode(false);
                    Refresh();
                }
            }
            else
            {
                ToggleAddMode(true);
            }
        }