Fix TypeError caused by calling int() on a NoneType

Error is:

```
Traceback (most recent call last):
  File "./cloudapp", line 24, in <module>
    main()
  File "./cloudapp", line 19, in main
    mainWidget = DropWidget()
  File "/home/samuel/code/py-cloudapp/droptarget.py", line 21, in __init__
    self.trayIcon = TrayIcon()
  File "/home/samuel/code/py-cloudapp/trayicon.py", line 20, in __init__
    self.apiHandle = CloudHandle()
  File "/home/samuel/code/py-cloudapp/cloudhandle.py", line 17, in __init__
    self.pdialog = PreferencesDialog()
  File "/home/samuel/code/py-cloudapp/preferences.py", line 18, in __init__
    self.retriveSettings()
  File "/home/samuel/code/py-cloudapp/preferences.py", line 70, in retriveSettings
    self.settings['list_size'] = int(self.obj.value('list_size'))
TypeError: int() argument must be a string or a number, not 'NoneType'
```