Zero nextScreen_ on screenmanager shutdown.

ui/screen.cpp

@@ -125,6 +125,7 @@ void ScreenManager::shutdown() {
 	}
 	stack_.clear();
 	delete nextScreen_;
+	nextScreen_ = 0;
 }
 
 void ScreenManager::push(Screen *screen, int layerFlags) {
@@ -174,4 +175,4 @@ void ScreenManager::processFinishDialog()
 		delete dialogFinished_;
 		dialogFinished_ = 0;
 	}
-}
+}