'nil' signal handler crashes the app

(I'm using lgi 0.9.1, the package provided with Ubuntu 16.10. Maybe the bug got fixed since then.)

When you install nil as a signal handler, the app crashes when it gets invoked. Here's an example:

#! /usr/bin/env lua

local lgi = require 'lgi'
local Gtk = lgi.Gtk

local w = Gtk.Window {
  title = "If you click the button, I'll crash",
  Gtk.Button { label = "click me!", id = "btn" },
  on_destroy = Gtk.main_quit,
}

local btn = w.child.btn
btn.on_clicked = nil    -- crash when clicked!

w:show_all()
Gtk.main()

(The bug is not specific to GtkButton, nor to on_clicked.)

BTW, I know there's no sense in assigning nil to a signal, as assignment adds a handler, not removes the previous one installed. I triggered the bug when I misspelled a function name. This kind of bugs (misspellings) can be hard to find (luacheck and strict.lua cannot always help).

Start with the Lua signal-assignment path exercised by btn.on_clicked = nil in the provided Gtk.Button example, and reproduce the crash by running the example and clicking the button. Done means assigning nil as a signal handler no longer crashes the application, including for signals other than on_clicked.