Currently, CraftTravelAgent will call s() on the passed-in WorldServer in order to set DEFAULT. However, s() will always return null at this point, because WorldServer.P will still be null, as it is set after the constructor is called. Instead, we set CraftTravelAgent.DEFAULT to the instance that is being constructed.
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.
