Handle the case where 'vtable-update-object' doesn't find old object

* lisp/emacs-lisp/vtable.el (vtable-update-object): If OLD-OBJECT
is not found, don't call ELT, since SEQ-POSITION may return nil.
(Bug#69664)

lisp/emacs-lisp/vtable.el

@@ -300,28 +300,28 @@ If it can't be found, return nil and don't move point."
         (error "Can't find the old object"))
       (setcar (cdr objects) object))
     ;; Then update the cache...
-    (let* ((line-number (seq-position (car (vtable--cache table)) old-object
-                                      (lambda (a b)
-                                        (equal (car a) b))))
-           (line (elt (car (vtable--cache table)) line-number)))
-      (unless line
-        (error "Can't find cached object"))
-      (setcar line object)
-      (setcdr line (vtable--compute-cached-line table object))
-      ;; ... and redisplay the line in question.
-      (save-excursion
-        (vtable-goto-object old-object)
-        (let ((keymap (get-text-property (point) 'keymap))
-              (start (point)))
-          (delete-line)
-          (vtable--insert-line table line line-number
-                               (nth 1 (vtable--cache table))
-                               (vtable--spacer table))
-          (add-text-properties start (point) (list 'keymap keymap
-                                                   'vtable table))))
-      ;; We may have inserted a non-numerical value into a previously
-      ;; all-numerical table, so recompute.
-      (vtable--recompute-numerical table (cdr line)))))
+    (if-let ((line-number (seq-position (car (vtable--cache table)) old-object
+                                        (lambda (a b)
+                                          (equal (car a) b))))
+             (line (elt (car (vtable--cache table)) line-number)))
+        (progn
+          (setcar line object)
+          (setcdr line (vtable--compute-cached-line table object))
+          ;; ... and redisplay the line in question.
+          (save-excursion
+            (vtable-goto-object old-object)
+            (let ((keymap (get-text-property (point) 'keymap))
+                  (start (point)))
+              (delete-line)
+              (vtable--insert-line table line line-number
+                                   (nth 1 (vtable--cache table))
+                                   (vtable--spacer table))
+              (add-text-properties start (point) (list 'keymap keymap
+                                                       'vtable table))))
+          ;; We may have inserted a non-numerical value into a previously
+          ;; all-numerical table, so recompute.
+          (vtable--recompute-numerical table (cdr line)))
+      (error "Can't find cached object in vtable"))))
 
 (defun vtable-remove-object (table object)
   "Remove OBJECT from TABLE.