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

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

@@ -164,6 +164,7 @@ public class PlayerSetter {
                             = DataSerializer.deserializeAdvancementData(data.getSerializedAdvancements());
 
                     if (Settings.useNativeImplementation) {
+                        Bukkit.getScheduler().runTaskAsynchronously(plugin, () -> {
                             try {
                                 nativeSyncPlayerAdvancements(player, advancementRecords);
                             } catch (Exception e) {
@@ -176,6 +177,7 @@ public class PlayerSetter {
                                 setPlayerAdvancements(player, advancementRecords, data);
                                 plugin.getLogger().log(Level.SEVERE, e.getMessage(), e);
                             }
+                        });
                     } else {
                         setPlayerAdvancements(player, advancementRecords, data);
                     }
@@ -313,9 +315,11 @@ public class PlayerSetter {
             }
         });
 
+        synchronized (playerAdvancements) {
             AdvancementUtils.markPlayerAdvancementsFirst(playerAdvancements);
             AdvancementUtils.ensureAllVisible(playerAdvancements);
         }
+    }
 
     /**
      * Update a player's advancements and progress to match the advancementData

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("\\.");