WPF/C#, Dealing With Data Access Layer and Business Logic Layer Using Entity Framework

public partial class AddCar : Window
    {
        RentACarEntities db = new RentACarEntities();
        public AddCar()
        {
            InitializeComponent();
            this.Loaded += new RoutedEventHandler(OnLoad);
        }
        void OnLoad(object sender,EventArgs e)
        {
            var items= new List<string>() { "Petrol","Diesel","CNG"};
            foreach(var item in items)
            {
                cmbfuel.Items.Add(item);
            }
        }

        void SaveData()
        {
            try
            {
                Car car = new Car();
                car.Type = txttype.Text;
                car.Model = txtmodel.Text;
                car.RegistrationNumber = "";
                car.Year = txtyear.Text;
                car.Fuel = cmbfuel.SelectedItem.ToString();
                car.RentPerDay = Convert.ToDouble(txtrent.Text);
                db.Cars.Add(car);
                if(db.SaveChanges()>0)
                MessageBox.Show("New Car Added Successfully");
            }
            catch(Exception ex)
            {
                MessageBox.Show("Error occured while adding new Car");
            }
        }
        private void Button_Click(object sender, RoutedEventArgs e)
        {
            SaveData();
        }

        private void Button_Click_1(object sender, RoutedEventArgs e)
        {
            this.Close();
        }
    }