Fix NPE involving WorldTypes when using CraftServer.createWorld(WorldCreator creator)

By: md-5 <md_5@bigpond.com>

paper-server/src/main/java/org/bukkit/craftbukkit/CraftServer.java

@@ -513,7 +513,7 @@ public final class CraftServer implements Server {
         ChunkGenerator generator = creator.generator();
         File folder = new File(name);
         World world = getWorld(name);
-        WorldType type = WorldType.a(creator.type().getName());
+        WorldType type = WorldType.a(creator.type().name());
 
         if (world != null) {
             return world;