The error trace is shown below.

Debugger entered--Lisp error: (wrong-type-argument char-or-string-p nil)
  quail-help("korean-hangul")
  eval((quail-help "korean-hangul") nil)
  eval-last-sexp-1(t)
  eval-last-sexp(t)
  eval-print-last-sexp()
  call-interactively(eval-print-last-sexp nil nil)
  recursive-edit()
  debug(error (wrong-type-argument char-or-string-p nil))
  quail-help("korean-hangul")
  eval((quail-help "korean-hangul") nil)
  eval-last-sexp-1(t)
  eval-last-sexp(t)
  eval-print-last-sexp()
  call-interactively(eval-print-last-sexp nil nil)
  recursive-edit()
  debug(error (void-function quail-help))
  (quail-help "korean-hangul")
  eval((quail-help "korean-hangul") nil)
  eval-last-sexp-1(t)
  eval-last-sexp(t)
  eval-print-last-sexp()
  call-interactively(eval-print-last-sexp nil nil)
  recursive-edit()
  debug(error (void-function quail-help))
  (quail-help (quote korean-hangul))
  eval((quail-help (quote korean-hangul)) nil)
  eval-last-sexp-1(t)
  eval-last-sexp(t)
  eval-print-last-sexp()
  call-interactively(eval-print-last-sexp nil nil)