bug#33294: xwidget-insert crashes Emacs

[Eli Zaretskii]

> From: Evgeny Zajcev <lg.zevlg@gmail.com>
> Date: Tue, 13 Nov 2018 14:43:40 +0300
> Cc: monnier@iro.umontreal.ca, rpluim@gmail.com, gazally@runbox.com, 
>       33294@debbugs.gnu.org
> 
>  > However, I noticed that Emacs without GUI (-nw -Q) continues to crash in 
> different place:
> 
>  Does it also crash with a valid Lisp call, i.e. when the buffer is not
>  empty?
> 
> Yeah, same behaviour with space inserted into buffer

Than this is a separate problem.  Looks like we need some flag to know
whether GTK has been initiaized (set in xg_initialize), and if not,
error out of make-xwidget and maybe xwidget_init_view.

I don't have access to an Emacs with xwidget support to test this; can
someone please provide a patch for that?