BUG: unexpected prompt for Recipients during Fcc

[Antoine Beaupré]

I'm having trouble writing long encrypted emails.

On startup, this works:

 1. start emacs
 2. start notmuch (M-x notmuch-hello)
 3. compose email ("c"), encrypted ("control-enter e")
 4. send ("control-c c")

But eventually, I get into this weird state. I originally thought it was
related with the `notmuch-draft-save-plaintext` prompt because in both
cases I had saved the long email before the issue was triggered. But now
i'm trying to reproduce with this:

 1. start emacs
 2. start notmuch
 3. compose email, encrypted
 4. save email ("control-x s")
 5. refuse to answer ("control-g") or say no ("n") to the "Really save
    and index an unencrypted copy?" question
 5. send

... but that, unexpectedly, works right now, which confuses me slightly.

The problematic behavior is that instead of working, the "send" step
does the following:

 1. email is sent
 2. the Fcc step unexpectedly prompts me for a "Recipient:" in the
    modeline
 3. enter a valid OpenPGP identity (I tried an email address and OpenPGP
    key)
 4. emacs fires up a `gpg --list-keys` process (according to ps) and
    then fails with "Wrong type argument: stringp, nil"
 5. email is not saved to disk

A workaround, at that stage, is to:

 1. save the email as a draft ("control-c p")
 2. exit emacs
 3. restart emacs
 4. find the draft ("s tag:draft")
 5. resume draft ("e")
 6. send the email (now a duplicate, but fcc will work now)

I'm a bit at a loss on why this is happening or how to trigger it. I
also don't have a debug backtrace of the problem, unfortunately, but I
promise to submit one when it happens again, because it's extremely
irritating. :) At least there's a way to not drop the email, even though
it means duplicate emails in the recipient's inbox.

This is on notmuch 0.28.4-1, Debian buster, Emacs 26.1, and
notmuch-config.el:

https://gitlab.com/anarcat/emacs-d/blob/master/notmuch-config.el
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