Fix LevelListRequestHandler.

This handler did not return list of strings but list of challenge levels, that is incorrect.
Not it should work correctly.

src/main/java/world/bentobox/challenges/ChallengesManager.java

@@ -1725,6 +1725,23 @@ public class ChallengesManager
     }
 
 
+    /**
+     * This method returns list of challenge levels in given gameMode.
+     * @param world for which levels must be searched.
+     * @return List with challengeLevel uniqueIds in given world.
+     */
+    public List<String> getLevelNames(@NonNull World world)
+    {
+        return this.islandWorldManager.getAddon(world).map(gameMode ->
+            this.levelCacheData.values().stream().
+                sorted(ChallengeLevel::compareTo).
+                filter(level -> level.matchGameMode(gameMode.getDescription().getName())).
+                map(ChallengeLevel::getUniqueId).
+                collect(Collectors.toList())).
+            orElse(Collections.emptyList());
+    }
+
+
     /**
      * Get challenge level by its challenge.
      *

src/main/java/world/bentobox/challenges/handlers/LevelListRequestHandler.java

@@ -52,7 +52,8 @@ public class LevelListRequestHandler extends AddonRequestHandler
             return Collections.emptyList();
         }
 
-        return this.addon.getChallengesManager().getLevels(Bukkit.getWorld((String) metaData.get("world-name")));
+        return this.addon.getChallengesManager().getLevelNames(
+            Bukkit.getWorld((String) metaData.get("world-name")));
     }