Cron bug with -1 value

[Carlos Agra]

When storing the period of the backup interval, the app crashes if there is no value selected.

Traceback (most recent call last):
  File "/home/carlos/hdlorean/gui/src/gui/Preferences.py", line 524, in applyB
    self.savePreferences(widget)
  File "/home/carlos/hdlorean/gui/src/gui/Preferences.py", line 549, in savePreferences
    self.__proxy.setPeriodically(self.__modBackEveryVal)
  File "/home/carlos/hdlorean/gui/src/gui/FrontendProxy.py", line 225, in setPeriodically
    self.__server.setPeriodically(period)
  File "/var/lib/python-support/python2.5/dbus/proxies.py", line 140, in __call__
    **keywords)
  File "/var/lib/python-support/python2.5/dbus/connection.py", line 607, in call_blocking
    message, timeout)
dbus.exceptions.DBusException: org.freedesktop.DBus.Python.TypeError: Traceback (most recent call last):
  File "/var/lib/python-support/python2.5/dbus/service.py", line 696, in _message_cb
    retval = candidate_method(self, *args, **keywords)
  File "/home/carlos/hdlorean/gui/src/hdloreand/watcher/BackendProxy.py", line 144, in setPeriodically
    self.__conf.setPeriodically(period)
  File "/home/carlos/hdlorean/gui/src/hdloreand/watcher/ConfigFileManager.py", line 611, in setPeriodically
    self.__watcher.updateCrontab()
  File "/home/carlos/hdlorean/gui/src/hdloreand/watcher/Watcher.py", line 424, in updateCrontab
    self.__cronHandler.addTask()
  File "/home/carlos/hdlorean/gui/src/hdloreand/watcher/CronHandler.py", line 125, in addTask
    tmp.write(task+self.__cronCommand+" #hdlorean\n")
TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'

Estaba terminando el asistente, pero me he dado cuenta de que no podía guardar el valor -1 para especificar que no quieres que haya backups cada x tiempo, y no me dejaba. Después me he dado cuenta de que el error estaba ya en preferencias de hecho.