add: return null; 
context:   this.getValue = function(prefName, defaultValue) {    var prefType = pref.getPrefType(prefName);    // underlying preferences object throws an exception if pref doesn't exist    if (prefType == pref.PREF_INVALID) {      return defaultValue;    }    switch (prefType) {      case pref.PREF_STRING:        return pref.getCharPref(prefName);      case pref.PREF_BOOL:        return pref.getBoolPref(prefName);      case pref.PREF_INT:        return pref.getIntPref(prefName);    }  }
meta: 4236	/local1/tlutelli/issta_data/temp/all_js/javascript/2007_temp/2007/4236/ef3e413efeb541ded3a628847a73e269323628ed/prefmanager.js/clean/src/chrome/chromeFiles/content/prefmanager.js