Bug 924009 - Add check for whether preferences are defined before checking for lightweight theme preference. r=margaret

--HG--
extra : rebase_source : cb9ad7312ad2ab0efdfa26454b5e8324b4123456

mobile/android/chrome/content/browser.js

@@ -7656,8 +7656,9 @@ var Distribution = {
     }
 
     // Apply a lightweight theme if necessary
-    if (prefs["lightweightThemes.isThemeSelected"])
+    if (prefs && prefs["lightweightThemes.isThemeSelected"]) {
       Services.obs.notifyObservers(null, "lightweight-theme-apply", "");
+    }
 
     let localizedString = Cc["@mozilla.org/pref-localizedstring;1"].createInstance(Ci.nsIPrefLocalizedString);
     let localizeablePrefs = aData["LocalizablePreferences"];