Servlets :: Shopping Cart - Not Add More Than One Item

I have following servlet to add items into cart -

import java.io.*;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import javax.servlet.RequestDispatcher;

[Code]...

One item gets added to the cart successfully. When adding the second item it gives a Null Pointer Exception at line 53

cart.add(item);