made appliance ignore destruct when no listeners

core/src/main/java/eu/mhsl/craftattack/spawn/core/appliance/Appliance.java

@@ -102,6 +102,7 @@ public abstract class Appliance {
     }
 
     public void destruct(@NotNull JavaPlugin plugin) {
+        if(this.listeners == null) return;
         this.listeners.forEach(HandlerList::unregisterAll);
     }