Always return a TravelAgent; Fixes BUKKIT-3541

Recent changes caused PlayerPortalEvent to suddenly return null
unexpectedly and could end up in NPEs resulting that did not before.
This commit addresses that situation by always ensuring a TravelAgent
instance is returned.

The TravelAgent for world 0 is returned arbitrarily in an effort to
compensate for plugins that are implementation dependent and expect some
form of a TravelAgent to be accessible in the event at all times.

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

@@ -1764,7 +1764,7 @@ public abstract class Entity {
             Location exit = exitWorld != null ? minecraftserver.getPlayerList().calculateTarget(enter, minecraftserver.getWorldServer(i)) : null;
             boolean useTravelAgent = exitWorld != null && !(this.dimension == 1 && exitWorld.dimension == 1); // don't use agent for custom worlds or return from THE_END
 
-            TravelAgent agent = exit != null ? (TravelAgent) ((CraftWorld) exit.getWorld()).getHandle().s() : null;
+            TravelAgent agent = exit != null ? (TravelAgent) ((CraftWorld) exit.getWorld()).getHandle().s() : org.bukkit.craftbukkit.CraftTravelAgent.DEFAULT; // return arbitrary TA to compensate for implementation dependent plugins 
             EntityPortalEvent event = new EntityPortalEvent(this.getBukkitEntity(), enter, exit, agent);
             event.useTravelAgent(useTravelAgent);
             event.getEntity().getServer().getPluginManager().callEvent(event);

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

@@ -451,7 +451,7 @@ public abstract class PlayerList {
             }
         }
 
-        TravelAgent agent = exit != null ? (TravelAgent) ((CraftWorld) exit.getWorld()).getHandle().s() : null;
+        TravelAgent agent = exit != null ? (TravelAgent) ((CraftWorld) exit.getWorld()).getHandle().s() : org.bukkit.craftbukkit.CraftTravelAgent.DEFAULT; // return arbitrary TA to compensate for implementation dependent plugins
         PlayerPortalEvent event = new PlayerPortalEvent(entityplayer.getBukkitEntity(), enter, exit, agent, cause);
         event.useTravelAgent(useTravelAgent);
         Bukkit.getServer().getPluginManager().callEvent(event);

src/main/java/org/bukkit/craftbukkit/CraftTravelAgent.java

@@ -9,12 +9,17 @@ import org.bukkit.TravelAgent;
 
 public class CraftTravelAgent extends PortalTravelAgent implements TravelAgent {
 
+    public static TravelAgent DEFAULT = null;
+
     private int searchRadius = 128;
     private int creationRadius = 16;
     private boolean canCreatePortal = true;
 
     public CraftTravelAgent(WorldServer worldserver) {
         super(worldserver);
+        if (DEFAULT == null && worldserver.dimension == 0) {
+            DEFAULT = (TravelAgent) worldserver.s();
+        }
     }
 
     public Location findOrCreate(Location target) {