Missing protection for `preferences.json` and `config.json`

[ncduy0303]

The User Guide mentioned warnings about editting the data files (incorrect format will reset all the data), but did not warn users about editting the preferences.json and config.json files that are also generated.

If some users set the content of preferences.json into:

{
  "guiSettings" : {
    "windowWidth" : 740.0,
    "windowHeight" : 600.0,
    "windowCoordinates" : null
  },
  "addressBookFilePath" : null,
  "eventsFilePath" : null,
  "financeFilePath" : null
}

and the config.json into:

{
  "logLevel" : "INFO",
  "userPrefsFilePath" : null
}

Problem: The application will not be able to start. A possible approriate behaviour for the application is to reset these files into default correct values rather than not being able to start.