Got an error when use `gkroam-rebuild-caches'

[Kinneyzhang]

Debugger entered--Lisp error: (wrong-type-argument integer-or-marker-p nil)
  get-text-property(nil title)
  button-get(nil title)....

The reason is: when the pages with headline links are not opened, button-at function cannot get the button by position correctly.