Fixes a Level addon crash on startup.

Level addon crashed at the startup if Visit or Warps addon were not installed. It happened because Level addon main class were implementing Listener interface.
To avoid it and fix the crash, I moved migration listener to a separate class.

Fixes #2012

src/main/java/world/bentobox/level/Level.java

@@ -13,8 +13,6 @@ import org.bukkit.Bukkit;
 import org.bukkit.World;
 import org.bukkit.configuration.InvalidConfigurationException;
 import org.bukkit.configuration.file.YamlConfiguration;
-import org.bukkit.event.EventHandler;
-import org.bukkit.event.Listener;
 import org.bukkit.plugin.Plugin;
 import org.eclipse.jdt.annotation.NonNull;
 import org.eclipse.jdt.annotation.Nullable;
@@ -22,7 +20,6 @@ import org.eclipse.jdt.annotation.Nullable;
 import world.bentobox.bentobox.api.addons.Addon;
 import world.bentobox.bentobox.api.addons.GameModeAddon;
 import world.bentobox.bentobox.api.configuration.Config;
-import world.bentobox.bentobox.api.events.BentoBoxReadyEvent;
 import world.bentobox.bentobox.api.user.User;
 import world.bentobox.bentobox.database.objects.Island;
 import world.bentobox.bentobox.util.Util;
@@ -38,6 +35,7 @@ import world.bentobox.level.config.BlockConfig;
 import world.bentobox.level.config.ConfigSettings;
 import world.bentobox.level.listeners.IslandActivitiesListeners;
 import world.bentobox.level.listeners.JoinLeaveListener;
+import world.bentobox.level.listeners.MigrationListener;
 import world.bentobox.level.objects.LevelsData;
 import world.bentobox.level.objects.TopTenData;
 import world.bentobox.level.requests.LevelRequestHandler;
@@ -50,7 +48,7 @@ import world.bentobox.warps.Warp;
  * @author tastybento
  *
  */
-public class Level extends Addon implements Listener {
+public class Level extends Addon {
 
     // The 10 in top ten
     public static final int TEN = 10;
@@ -112,7 +110,8 @@ public class Level extends Addon implements Listener {
         // Register listeners
         this.registerListener(new IslandActivitiesListeners(this));
         this.registerListener(new JoinLeaveListener(this));
-        this.registerListener(this);
+        this.registerListener(new MigrationListener(this));
+
         // Register commands for GameModes
         registeredGameModes.clear();
         getPlugin().getAddonsManager().getGameModeAddons().stream()
@@ -174,7 +173,7 @@ public class Level extends Addon implements Listener {
         this.getAddonByName("Visit").ifPresentOrElse(addon ->
         {
             this.visitHook = (VisitAddon) addon;
-            this.log("Likes Addon hooked into Visit addon.");
+            this.log("Level Addon hooked into Visit addon.");
         }, () ->
         {
             this.visitHook = null;
@@ -184,7 +183,7 @@ public class Level extends Addon implements Listener {
         this.getAddonByName("Warps").ifPresentOrElse(addon ->
         {
             this.warpHook = (Warp) addon;
-            this.log("Likes Addon hooked into Warps addon.");
+            this.log("Level Addon hooked into Warps addon.");
         }, () ->
         {
             this.warpHook = null;
@@ -217,39 +216,6 @@ public class Level extends Addon implements Listener {
         return comparisonResult;
     }
 
-    @EventHandler
-    public void onBentoBoxReady(BentoBoxReadyEvent e) {
-        // Perform upgrade check
-        manager.migrate();
-        // Load TopTens
-        manager.loadTopTens();
-        /*
-         * DEBUG code to generate fake islands and then try to level them all.
-        Bukkit.getScheduler().runTaskLater(getPlugin(), () -> {
-            getPlugin().getAddonsManager().getGameModeAddons().stream()
-            .filter(gm -> !settings.getGameModes().contains(gm.getDescription().getName()))
-            .forEach(gm -> {
-                for (int i = 0; i < 1000; i++) {
-                    try {
-                        NewIsland.builder().addon(gm).player(User.getInstance(UUID.randomUUID())).name("default").reason(Reason.CREATE).noPaste().build();
-                    } catch (IOException e1) {
-                        // TODO Auto-generated catch block
-                        e1.printStackTrace();
-                    }
-                }
-            });
-            // Queue all islands DEBUG
-
-            getIslands().getIslands().stream().filter(Island::isOwned).forEach(is -> {
-
-                this.getManager().calculateLevel(is.getOwner(), is).thenAccept(r ->
-                log("Result for island calc " + r.getLevel() + " at " + is.getCenter()));
-
-            });
-       }, 60L);*/
-    }
-
-
     private void registerPlaceholders(GameModeAddon gm) {
         if (getPlugin().getPlaceholdersManager() == null) return;
         // Island Level

src/main/java/world/bentobox/level/LevelsManager.java

@@ -35,7 +35,6 @@ import world.bentobox.level.objects.TopTenData;
 public class LevelsManager {
     private static final String INTOPTEN = "intopten";
     private static final TreeMap<BigInteger, String> LEVELS;
-    private static final int[] SLOTS = new int[] {4, 12, 14, 19, 20, 21, 22, 23, 24, 25};
     private static final BigInteger THOUSAND = BigInteger.valueOf(1000);
     static {
         LEVELS = new TreeMap<>();
@@ -45,7 +44,7 @@ public class LevelsManager {
         LEVELS.put(THOUSAND.pow(3), "G");
         LEVELS.put(THOUSAND.pow(4), "T");
     }
-    private Level addon;
+    private final Level addon;
 
     // Database handler for level data
     private final Database<IslandLevels> handler;
@@ -341,7 +340,7 @@ public class LevelsManager {
     /**
      * Loads all the top tens from the database
      */
-    void loadTopTens() {
+    public void loadTopTens() {
         topTenLists.clear();
         Bukkit.getScheduler().runTaskAsynchronously(addon.getPlugin(), () -> {
             addon.log("Generating rankings");

src/main/java/world/bentobox/level/listeners/MigrationListener.java

@@ -0,0 +1,61 @@
+//
+// Created by BONNe
+// Copyright - 2022
+//
+
+
+package world.bentobox.level.listeners;
+
+
+import org.bukkit.event.EventHandler;
+import org.bukkit.event.Listener;
+
+import world.bentobox.bentobox.api.events.BentoBoxReadyEvent;
+import world.bentobox.level.Level;
+
+
+/**
+ * This listener checks when BentoBox is ready and then tries to migrate Levels addon database, if it is required.
+ */
+public class MigrationListener implements Listener
+{
+    public MigrationListener(Level addon)
+    {
+        this.addon = addon;
+    }
+
+    @EventHandler
+    public void onBentoBoxReady(BentoBoxReadyEvent e) {
+        // Perform upgrade check
+        this.addon.getManager().migrate();
+        // Load TopTens
+        this.addon.getManager().loadTopTens();
+        /*
+         * DEBUG code to generate fake islands and then try to level them all.
+        Bukkit.getScheduler().runTaskLater(getPlugin(), () -> {
+            getPlugin().getAddonsManager().getGameModeAddons().stream()
+            .filter(gm -> !settings.getGameModes().contains(gm.getDescription().getName()))
+            .forEach(gm -> {
+                for (int i = 0; i < 1000; i++) {
+                    try {
+                        NewIsland.builder().addon(gm).player(User.getInstance(UUID.randomUUID())).name("default").reason(Reason.CREATE).noPaste().build();
+                    } catch (IOException e1) {
+                        // TODO Auto-generated catch block
+                        e1.printStackTrace();
+                    }
+                }
+            });
+            // Queue all islands DEBUG
+
+            getIslands().getIslands().stream().filter(Island::isOwned).forEach(is -> {
+
+                this.getManager().calculateLevel(is.getOwner(), is).thenAccept(r ->
+                log("Result for island calc " + r.getLevel() + " at " + is.getCenter()));
+
+            });
+       }, 60L);*/
+    }
+
+
+    private final Level addon;
+}