问org.springframework.expression.spel.SpelEvaluationException: EL1007E:在null上找不到属性或字段“items”

EN

     org.thymeleaf.exceptions.TemplateProcessingException: Exception evaluating SpringEL expression: "!session.shoppingCart.items"
org.springframework.expression.spel.SpelEvaluationException: EL1007E: Property or field 'items' cannot be found on null

  <div th:if="${!session.shoppingCart.items}">
    <tr th:each="item : ${session.shoppingCart.items}">
        <td th:text="${item.book.id}"></td>
            <td th:text="${item.book.title}"></td>
            <td><span th:text="${item.book.price}"></span>cash</td>
            <td>
                <form action="#" th:action="@{/cart/update}" method="post">
                    <input type="hidden" th:value="${item.book.id}" name="id"/>
                    <input type="number" min="1" th:value="${item.quantity}" name="qty"/>
                    <button type="submit">UPDATE</button>
                </form>
            </td>
            <td><span th:text="${item.subTotal}"></span>cash</td>
            <td>
                <form action="#" th:action="@{/cart/remove}" method="post">
                    <input type="hidden" th:value="${item.book.id}" name="id"/>
                    <button type="submit">remove</button>
                </form>
            </td>
        </tr>
    </div>