Try not_nil!

Signal::INT.trap do |x|
  if fg = Jobs.manager.fg
    fg.kill(Signal::INT)
  else
    puts "Got SIGINT, but there is no process in the foreground."
  end
end

# or

Signal::INT.trap do |x|
  if !Jobs.manager.fg
    puts "Got SIGINT, but there is no process in the foreground."
  else
    Jobs.manager.fg.not_nil!.kill(Signal::INT)
  end
end