I get the error in the subject with Vim 8.0.0071 under macOS. To reproduce, create the following vimrc_minimal:

set nocompatible

fun! SendToBuffer(cmd)
  let l:job = job_start(a:cmd, {
        \ "close_cb": "CloseCallback",
        \ "exit_cb": "ExitCallback",
        \ "out_io": "buffer"})
  execute  "bo split +buffer".ch_getbufnr(l:job, "out")
  wincmd p
endf

fun! CloseCallback(channel)
  call job_status(ch_getjob(a:channel))
endf

fun! ExitCallback(job, status)
  exe "echomsg" (a:status == 0 ? "'Success!'" : "'Failed.'")
endf

Then, run vim -u vimrc_minimal and execute:

:call SendToBuffer('ls')

Sometimes, the error is not raised: in that case, try the command repeatedly. This is what I eventually see in messages:

Success!
Error detected while processing function CloseCallback:
line    1:
E916: not a valid job

The specific command shouldn't matter (in fact, I first saw the error using git push), although some commands seem to cause the error more frequently than others.

[Edit: fixed typo in Vim version]