Don't try to teleport to a null world. Fixes BUKKIT-5145

src/main/java/net/minecraft/server/Entity.java

@@ -1765,7 +1765,7 @@ public abstract class Entity {
             EntityPortalEvent event = new EntityPortalEvent(this.getBukkitEntity(), enter, exit, agent);
             event.useTravelAgent(useTravelAgent);
             event.getEntity().getServer().getPluginManager().callEvent(event);
-            if (event.isCancelled() || event.getTo() == null || !this.isAlive()) {
+            if (event.isCancelled() || event.getTo() == null || event.getTo().getWorld() == null || !this.isAlive()) {
                 return;
             }
             exit = event.useTravelAgent() ? event.getPortalTravelAgent().findOrCreate(event.getTo()) : event.getTo();