*Backtrace*.txt

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  get-file-buffer(nil)
  org-find-base-buffer-visiting(nil)
  org-get-agenda-file-buffer(nil)
  org-refile-get-targets(nil (#("foo" 0 3 (fontified t line-prefix "" wrap-prefix #("* " 0 2 (face org-indent)) face org-level-1))))
  org-refile-get-location("Refile subtree \"foo\" to" nil confirm nil)
  org-refile(nil)
  call-interactively(org-refile record nil)
  command-execute(org-refile record)
  helm-M-x(nil "org-refile")
  call-interactively(helm-M-x nil nil)
  command-execute(helm-M-x)