conflict with ftdplugin enabled?

[sesselastronaut]

Don't sure if this is a 'plain' neocomplete issue as I use neocomplete and vimtex together. If the following line in my vimrc is uncommented: let b:did_ftplugin = 1 and I try to use the completion of e.g. \cite{ in a .tex file by pressing <C-x><C-o> vim returns: E764: Option 'omnifunc' is not set With the above line in comments it works as expected and brings up the options from the .bib file.

This occurs on Platform: x86_64-pc-linux-gnu (64-bit) Running under: Debian GNU/Linux 9 (stretch) with VIM - Vi IMproved 8.0 (2016 Sep 12, compiled Mar 07 2017 03:33:23) Included patches: 1-197, 322, 377-378

This is a MWE vimrc:

filetype plugin indent on
syntax enable
"" uncomment/comment the following line to reproduce
let b:did_ftplugin = 1
let &rtp  = '~/.vim/bundle/vimtex,' . &rtp
let &rtp .= ',~/.vim/bundle/vimtex/after'
let g:tex_flavor = 'latex'
let g:neocomplete#enable_at_startup = 1
let g:neocomplete#enable_camel_case = 1
let g:neocomplete#enable_auto_delimiter = 1
let g:neocomplete#enable_auto_close_preview = 1
let g:neocomplete#keyword_patterns = {}
let g:neocomplete#sources#omni#input_patterns = {}
let g:neocomplete#keyword_patterns.tex = '[a-zA-ZæÆøØåÅ][0-9a-zA-ZæÆøØåÅ]\+'
let g:neocomplete#sources#omni#input_patterns.tex =
      \ '\v\\\a*(ref|cite)\a*([^]]*\])?\{([^}]*,)*[^}]*'

to reproduce

1. place the above vimrc in a folder e.g. as test.vimrc
2. setup a test.tex:

   \bibliographystyle{amsplain}
   \begin{document}
   \cite{
   \bibliography{TeX}
   \end{document}

   and a TeX.bib next to it:

   @book{diller97,
   author = "Antoni Diller",
   title = "{\LaTeX} Line by Line",
   year = "1997",
   edition = "1st",
   publisher = "Wiley",
   address = "Chichester, England"
   }

3. start vim vim -u test.vimrc test.tex
4. go the end of the line with \cite{ and press <C-x> <C-o> in insert mode

[Shougo]

ftplugin set omnifunc.

let b:did_ftplugin = 1

It disables ftplugin, so omnifunc feature is disabled.