def pid
      return nil unless is_window?
      process_id = 0.chr * 4
      GetWindowThreadProcessId(handle, process_id)
      process_id = process_id.unpack('L').first
    end
def pid
      return nil unless is_window?
      process_id = 0.chr * 4
      GetWindowThreadProcessId(handle, process_id)
      process_id = process_id.unpack('L').first
    end
The identifier (pid) of the process that created the window

 @return [Integer] process id if the window exists, otherwise nil