I get the following error when I do org-mobile-pull. The backtrace are
these following 4 lines.

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  org-mobile-pull()
  call-interactively(org-mobile-pull t nil)
  execute-extended-command(nil)
  call-interactively(execute-extended-command nil nil)

I am using 7.8.03 version of org-mode on 24.0.91.1 emacs.
Could you anyone help to get a verbose trace enough to understand the
problem
and fix it?

Thanks.