无法理解为什么我得到一个空指针错误

我对Java编码很陌生，对堆栈交换也很陌生。我一直在尝试做这个汽车经销计划。尝试将汽车或自行车添加到车辆列表中时，我一直收到空指针错误，但我似乎无法弄清楚为什么，我几乎肯定是我把东西弄混了，放在错误的位置，但是我无法弄清楚哪去了。我使用一个驱动程序，一个Vehicles超类和Cars and Bikes作为两个继承自Vehicles的子类。抱歉，如果对任何内容不清楚，我们将不胜感激。谢谢！

public class BMWdriver {

ArrayList<Vehicles> list;

public void Driver() {
    list = new ArrayList<Vehicles>();

}

public void inputCarDetails() {
    Scanner scan = new Scanner(System.in);
    String model, colour, fuelType, layout, frame, vehicleType;
    int doors, stock, displacement, topSpeed, stroke, noSeats, noVehicles;
    double price, fuelMpg;
    boolean sunroof;
    Vehicles car;

    System.out.println("----Entering car details----");
    System.out.println("\nEnter model");
    model = scan.nextLine();
    System.out.println("Enter Price");
    price = scan.nextDouble();
    scan.nextLine();
    System.out.println("Enter colour");
    colour = scan.nextLine();
    System.out.println("Enter no. in stock");
    stock = scan.nextInt();
    System.out.println("Enter MPG");
    fuelMpg = scan.nextDouble();
    System.out.println("Enter displacement");
    displacement = scan.nextInt();
    System.out.println("Enter top speed");
    topSpeed = scan.nextInt();
    System.out.println("Enter no. of doors");
    doors = scan.nextInt();
    System.out.println("Enter fuel type");
    fuelType = scan.nextLine();
    System.out.println("Enter wheel layout");
    layout = scan.nextLine();
    System.out.println("Enter sunroof (true/false)");
    sunroof = scan.nextBoolean();

    car = new Cars(model, price, colour, stock, fuelMpg, displacement, topSpeed, doors, fuelType, layout, sunroof);
    list.add(car);
}

public void inputBikeDetails() {
    Scanner scan = new Scanner(System.in);
    String model, colour, fuelType, layout, frame, vehicleType;
    int doors, stock, displacement, topSpeed, stroke, noSeats, noVehicles;
    double price, fuelMpg;
    boolean sunroof;
    Vehicles bike;

    System.out.println("----Entering bike details----");
    System.out.println("\nEnter model");
    model = scan.nextLine();
    System.out.println("Enter Price");
    price = scan.nextDouble();
    scan.nextLine();
    System.out.println("Enter colour");
    colour = scan.nextLine();
    System.out.println("Enter no. in stock");
    stock = scan.nextInt();
    System.out.println("Enter MPG");
    fuelMpg = scan.nextDouble();
    System.out.println("Enter displacement");
    displacement = scan.nextInt();
    System.out.println("Enter top speed");
    topSpeed = scan.nextInt();
    System.out.println("Enter engine stroke");
    stroke = scan.nextInt();
    System.out.println("Enter no. of seats");
    noSeats = scan.nextInt();
    System.out.print("Enter the frame type");
    frame = scan.nextLine();

    bike = new Bikes(model, price, colour, stock, fuelMpg, displacement, topSpeed, stroke, noSeats, frame);
    list.add(bike);

}

public static void main(String[] args) // main method
{
    BMWdriver driver = new BMWdriver();
    driver.startMenu();
    driver.inputCarDetails();
    driver.inputBikeDetails();
}
}