Fix assertion when going back fron network to normal kart selection screen

src/states_screens/kart_selection.cpp

@@ -321,8 +321,6 @@ void KartSelectionScreen::init()
     Widget* placeholder = getWidget("playerskarts");
     assert(placeholder != NULL);
 
-    // FIXME : The reserved id value is -1 when we switch from KSS to NKSS and vice-versa
-
     m_dispatcher->setRootID(placeholder->m_reserved_id);
 
     g_root_id = placeholder->m_reserved_id;

src/states_screens/network_kart_selection.hpp

@@ -54,6 +54,12 @@ public:
     // ------------------------------------------------------------------------
     virtual bool playerQuit(StateManager::ActivePlayer* player) OVERRIDE
                                                                { return true; }
+     /** \brief implement callback from parent class GUIEngine::Screen */
+    virtual void tearDown() OVERRIDE
+    {
+        m_must_delete_on_back = true;
+        KartSelectionScreen::tearDown();
+    }
 
 };