Note_column programming errors

[Dan Eble]

> I'm not top posting.
% This sample produces these warnings:
%   programming error: note column without heads and stem
%   programming error: Note_column without first_head()
%
% If this deserves a ticket, please create one.
% I intend to try to fix it myself.
\version "2.19.13"

\layout {
  \context {
    \Staff
    \accepts "HiddenVoice"
  }

  \context {
    \name "HiddenVoice"
    \type "Engraver_group"
    \consists "Note_heads_engraver"
    \hide NoteHead
  }
}

\score {
  <<
    \new Staff <<
      \new Voice { \relative c'' { c4 r4 } }
      \new HiddenVoice { \relative c' { a4. g8 } }
    >>
  >>
}