#vulnerable code 
                             @Override
	public HttpExchangeBody goBack(int steps) {

		List<String> stack = session(SESSION_PAGE_STACK, null);
		String dest = !stack.isEmpty() ? stack.get(stack.size() - 1) : "/";

		for (int i = 0; i < steps; i++) {
			if (stack != null && !stack.isEmpty()) {
				stack.remove(stack.size() - 1);
				if (!stack.isEmpty()) {
					dest = stack.remove(stack.size() - 1);
				}
			}
		}

		return redirect(dest);
	} 
                            #location 5 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                             @Override
	public HttpExchangeBody goBack(int steps) {
		String dest = "/";
		List<String> stack = session(SESSION_PAGE_STACK, null);

		if (stack != null) {
			if (!stack.isEmpty()) {
				dest = stack.get(stack.size() - 1);
			}

			for (int i = 0; i < steps; i++) {
				if (!stack.isEmpty()) {
					stack.remove(stack.size() - 1);
					if (!stack.isEmpty()) {
						dest = stack.remove(stack.size() - 1);
					}
				}
			}
		}

		return redirect(dest);
	}