Perform some validations when clicking on list item in addons screen

git-svn-id: svn+ssh://svn.code.sf.net/p/supertuxkart/code/main/trunk@9781 178a84e3-b1eb-0310-8ba1-8eac791a3b58

src/states_screens/addons_screen.cpp

@@ -303,7 +303,7 @@ void AddonsScreen::eventCallback(GUIEngine::Widget* widget,
             getWidget<GUIEngine::ListWidget>("list_addons");
         std::string id = list->getSelectionInternalName();
 
-        if (!id.empty())
+        if (!id.empty() && addons_manager->getAddon(id) != NULL)
         {
             m_selected_index = list->getSelectionID();
             new AddonsLoading(0.8f, 0.8f, id);