Maybe fix https://github.com/HarvelsX/HuskSync/issues/1;

bukkit/src/main/java/me/william278/husksync/bukkit/util/PlayerSetter.java

@@ -164,18 +164,20 @@ public class PlayerSetter {
                             = DataSerializer.deserializeAdvancementData(data.getSerializedAdvancements());
 
                     if (Settings.useNativeImplementation) {
-                        try {
-                            nativeSyncPlayerAdvancements(player, advancementRecords);
-                        } catch (Exception e) {
-                            plugin.getLogger().log(Level.WARNING,
-                                    "Your server does not support a native implementation of achievements synchronization");
-                            plugin.getLogger().log(Level.WARNING,
-                                    "Your server version {0}. Please disable using native implementation!", Bukkit.getVersion());
-
-                            Settings.useNativeImplementation = false;
-                            setPlayerAdvancements(player, advancementRecords, data);
-                            plugin.getLogger().log(Level.SEVERE, e.getMessage(), e);
-                        }
+                        Bukkit.getScheduler().runTaskAsynchronously(plugin, () -> {
+                            try {
+                                nativeSyncPlayerAdvancements(player, advancementRecords);
+                            } catch (Exception e) {
+                                plugin.getLogger().log(Level.WARNING,
+                                        "Your server does not support a native implementation of achievements synchronization");
+                                plugin.getLogger().log(Level.WARNING,
+                                        "Your server version {0}. Please disable using native implementation!", Bukkit.getVersion());
+
+                                Settings.useNativeImplementation = false;
+                                setPlayerAdvancements(player, advancementRecords, data);
+                                plugin.getLogger().log(Level.SEVERE, e.getMessage(), e);
+                            }
+                        });
                     } else {
                         setPlayerAdvancements(player, advancementRecords, data);
                     }
@@ -313,8 +315,10 @@ public class PlayerSetter {
             }
         });
 
-        AdvancementUtils.markPlayerAdvancementsFirst(playerAdvancements);
-        AdvancementUtils.ensureAllVisible(playerAdvancements);
+        synchronized (playerAdvancements) {
+            AdvancementUtils.markPlayerAdvancementsFirst(playerAdvancements);
+            AdvancementUtils.ensureAllVisible(playerAdvancements);
+        }
     }
 
     /**

bukkit/src/main/java/me/william278/husksync/bukkit/util/nms/MinecraftVersionUtils.java

@@ -13,7 +13,7 @@ public class MinecraftVersionUtils {
     public final static String SERVER_VERSION = Bukkit.getBukkitVersion().split("-")[0];
 
     public static int compare(String version) {
-        if (version == null) return 1;
+        if (version == null || SERVER_VERSION == null) return 1;
 
         String[] as = SERVER_VERSION.split("\\.");
         String[] bs = version.split("\\.");