lisp-在访问控制模型中销毁obj时，“car: contract violation expected: pair? given：()”

(car ()) ; 错误：contract violation expected: pair? given: ()

(if (not (null list))
    (car list)
    nil) ; 或者其他适当的默认值

(defstruct my-object type data)

(defvar *obj* (make-my-object :type 'example :data (list 1 2 3)))

(defun destroy-object (obj)
  (when obj
    (setf *obj* nil)
    (car (my-object-data obj)))) ; 错误：如果*obj*已经被设置为nil，这里会出错

(defun destroy-object (obj)
  (when obj
    (let ((data (my-object-data obj)))
      (setf *obj* nil)
      data)))

(defun process-list (lst)
  (if (null lst)
      nil
      (cons (car lst) (process-list (cdr lst))))) ; 如果lst在某个点是()，这里会出错

(defun process-list (lst)
  (if (null lst)
      nil
      (cons (car lst) (process-list (cdr lst)))))