fix null pointer when playing tutorial

racevisionGame/src/main/java/visualiser/Controllers/HostGameController.java

@@ -80,6 +80,9 @@ public class HostGameController extends Controller {
      */
     public void alertMatchBrowser(){
         try{
+            if (matchBrowserInterface == null){
+                return;//private game
+            }
             matchBrowserInterface.startSendingHostData(App.game.getHostedGameData(), udpSocket);
         }catch (IOException e){
             System.err.println("failed to send out hosted game info");