Fix NPE on weather change in a game world if the game has not starte…

…d yet. See #538

src/main/java/de/erethon/dungeonsxl/world/DWorldListener.java

@@ -187,6 +187,9 @@ public void onWeatherChange(WeatherChangeEvent event) {
             event.setCancelled(true);
         } else if (dWorld instanceof DGameWorld) {
             Game game = Game.getByGameWorld((DGameWorld) dWorld);
+            if (game == null || game.getRules() == null) {
+                return;
+            }
             Boolean raining = game.getRules().isRaining();
             if (raining == null) {
                 return;