Don't call shutdown on the UI module if it isn't implemented

lib/main.js

@@ -123,7 +123,7 @@ require("sdk/system/unload").when(function unload(reason) {
   Preferences.reset(PREFS.PROXY_AUTOCONFIG_URL);
   Preferences.reset(PREFS.PROXY_TYPE);
 
-  if (gUI) {
+  if (gUI.shutdown) {
     gUI.shutdown(reason);
   }
 });