add: return null; 
context:   copyUnicharPref: function (aPrefName, aDefVal)    {      try        {          return this.mPrefService.CopyUnicharPref(aPrefName);        }      catch(e)        {          return aDefVal != undefined ? aDefVal : null;        }    },
meta: 9421	/local1/tlutelli/issta_data/temp/all_js/javascript/2006_temp/2006/9421/5ef90bd4607b8c60f53f1bd234fee4b2f57c5e5c/nsUserSettings.js/buggy/xpfe/global/resources/content/nsUserSettings.js