Java web programming, Null Pointer exception

When I run the Java servlet for the first time on the server, all the web pages work fine and there are no problems. But when I stop the server, restart it and start the servlet again, there is a null pointer exception on one page. I tried to print something where this error occurred, but when I write System.out.printl("something")there and run the servlet again (rebooting the server many times), a throw no longer occurs.

Does anyone help solve this problem?

Here is the doPost method where the exception is thrown

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    ShoppingCart cart = (ShoppingCart)request.getSession().getAttribute("Cart");
    ProductCatalog pc = (ProductCatalog) request.getServletContext().getAttribute("Product Catalog");
    String id = request.getParameter("productID");
    if(id != null){
        Product pr = pc.getProduct(id);
        cart.addItem(pr); // here is null pointer exception
    }
    RequestDispatcher dispatch = request.getRequestDispatcher("shopping-cart.jsp");
    dispatch.forward(request, response);
}

and here is the basket class:

ConcurrentHashMap personal items

/** Creates new Shopping Cart */
public ShoppingCart(){
    items = new ConcurrentHashMap<Product, Integer>();
}

/** Adds a product having this id into cart and increases quantity. */
//This method is called after "add to cart" button is clicked. 
public void addItem(Product pr){
    System.out.println(pr.getId() + " sahdhsaihdasihdasihs");
    if(items.containsKey(pr)){
        int quantity = items.get(pr) + 1;
        items.put(pr, quantity);
    } else {
        items.put(pr, 1);
    }
}

/** 
 * Adds a product having this id into cart and 
 * increases quantity with the specified number. 
 * If quantity is zero, we remove this product from cart.
 */
//This method is called many times after "update cart" button is clicked. 
public void updateItem(Product pr, int quantity){
    if(quantity == 0)items.remove(pr);
    else items.put(pr, quantity);
}

/** Cart iterator */
public Iterator<Product> cartItems(){
    return items.keySet().iterator();
}

/** Returns quantity of this product */
public int getQuantity(Product pr){
    return items.get(pr);
}