class Jobs
  MANAGER = new

  def self.manager
    MANAGER
  end

  def initialize(@fg : Process? = nil)
  end

  property fg
end

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

Gives the following error:
Error in src/crysh.cr:27: undefined method 'kill' for Nil (compile-time type is (Process | Nil))

    Jobs.manager.fg.kill(Signal::INT)
                    ^~~~