fixing some stuff

.gitignore

@@ -1,2 +1,3 @@
 /target/
 /.idea/
+/CoinsEngine.iml

src/main/java/su/nightexpress/coinsengine/tops/TopManager.java

@@ -34,7 +34,14 @@ public TopManager(@NotNull CoinsEnginePlugin plugin) {
     @Override
     protected void onLoad() {
         if (Config.TOPS_USE_GUI.get()) {
-            this.topMenu = this.addMenu(new TopMenu(this.plugin, this), Config.DIR_MENU, "leaderboard.yml");
+            try {
+                // checking whether symbol MenuType has existence because it is an Experimental class
+                // or just adapt each menu windows for each version like MythicMobs?
+                Class<?> menuTypeClass = Class.forName("org.bukkit.inventory.MenuType");
+                this.topMenu = this.addMenu(new TopMenu(this.plugin, this), Config.DIR_MENU, "leaderboard.yml");
+            } catch (ClassNotFoundException e) {
+                this.plugin.error("Couldn't enabling top menu because your server's Minecraft version is not supported.");
+            }
         }
 
         this.addListener(new TopsListener(this.plugin, this));

src/main/java/su/nightexpress/coinsengine/user/UserBalance.java

@@ -4,16 +4,16 @@
 import su.nightexpress.coinsengine.api.CoinsEngineAPI;
 import su.nightexpress.coinsengine.api.currency.Currency;
 
-import java.util.HashMap;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.function.Consumer;
 
 public class UserBalance {
 
     private final Map<String, Double> balanceMap;
 
     public UserBalance() {
-        this(new HashMap<>());
+        this(new ConcurrentHashMap<>());
     }
 
     public UserBalance(@NotNull Map<String, Double> balanceMap) {