Add NULL protection in challenge and level loading.

Remove Challenge and Level saving on server stop, as it is done in GUI after editing. Reduce load on database.
Use saveAsync, as saveObject is deprecated.

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

@@ -206,11 +206,15 @@ public void load()
         }
 
         this.playerCacheData.clear();
-        loadAndValidate();
+        this.loadAndValidate();
     }
 
 
-    private void loadAndValidate() {
+    /**
+     * This method loads and validates all challenges and levels.
+     */
+    private void loadAndValidate()
+    {
         this.challengeDatabase.loadObjects().forEach(this::loadChallenge);
         this.levelDatabase.loadObjects().forEach(this::loadLevel);
         // this validate challenge levels
@@ -233,7 +237,7 @@ public void reload()
         //this.levelDatabase = new Database<>(addon, ChallengeLevel.class);
         //this.playersDatabase = new Database<>(addon, ChallengesPlayerData.class);
 
-        loadAndValidate();
+        this.loadAndValidate();
     }
 
 
@@ -263,6 +267,18 @@ public boolean loadChallenge(@NonNull Challenge challenge,
             User user,
             boolean silent)
     {
+        // This may happen if database somehow failed to load challenge and return
+        // null as input.
+        if (challenge == null)
+        {
+            if (!silent)
+            {
+                user.sendMessage("load-error", "[value]", "NULL");
+            }
+
+            return false;
+        }
+
         if (this.challengeCacheData.containsKey(challenge.getUniqueId()))
         {
             if (!overwrite)
@@ -324,6 +340,18 @@ public boolean loadLevel(@NonNull ChallengeLevel level,
             User user,
             boolean silent)
     {
+        // This may happen if database somehow failed to load challenge and return
+        // null as input.
+        if (level == null)
+        {
+            if (!silent)
+            {
+                user.sendMessage("load-error", "[value]", "NULL");
+            }
+
+            return false;
+        }
+
         if (!this.isValidLevel(level))
         {
             if (user != null)
@@ -474,11 +502,8 @@ private boolean isValidLevel(@NonNull ChallengeLevel level)
         {
             if (!this.challengeCacheData.containsKey(uniqueID))
             {
-                if (this.challengeDatabase.objectExists(uniqueID))
-                {
-                    this.loadChallenge(this.challengeDatabase.loadObject(uniqueID));
-                }
-                else
+                if (!this.challengeDatabase.objectExists(uniqueID) ||
+                    !this.loadChallenge(this.challengeDatabase.loadObject(uniqueID), false, null, true))
                 {
                     this.addon.logError("Cannot find " + uniqueID + " challenge for " + level.getUniqueId());
                     return false;
@@ -824,8 +849,11 @@ private void migratePlayers(World world)
      */
     public void save()
     {
-        this.saveChallenges();
-        this.saveLevels();
+        // Challenges and Levels are saved on modifications only to avoid issues with
+        // NULL's in data after interrupting server while in saving stage.
+        // this.saveChallenges();
+        // this.saveLevels();
+
         this.savePlayersData();
     }
 
@@ -835,7 +863,7 @@ public void save()
      */
     private void saveChallenges()
     {
-        this.challengeCacheData.values().forEach(this.challengeDatabase::saveObject);
+        this.challengeCacheData.values().forEach(this::saveChallenge);
     }
 
 
@@ -845,7 +873,7 @@ private void saveChallenges()
      */
     public void saveChallenge(Challenge challenge)
     {
-        this.challengeDatabase.saveObject(challenge);
+        this.challengeDatabase.saveObjectAsync(challenge);
     }
 
 
@@ -854,7 +882,7 @@ public void saveChallenge(Challenge challenge)
      */
     private void saveLevels()
     {
-        this.levelCacheData.values().forEach(this.levelDatabase::saveObject);
+        this.levelCacheData.values().forEach(this::saveLevel);
     }
 
 
@@ -864,7 +892,7 @@ private void saveLevels()
      */
     public void saveLevel(ChallengeLevel level)
     {
-        this.levelDatabase.saveObject(level);
+        this.levelDatabase.saveObjectAsync(level);
     }
 
 
@@ -873,7 +901,7 @@ public void saveLevel(ChallengeLevel level)
      */
     private void savePlayersData()
     {
-        this.playerCacheData.values().forEach(this.playersDatabase::saveObject);
+        this.playerCacheData.values().forEach(this.playersDatabase::saveObjectAsync);
     }
 
 
@@ -902,7 +930,7 @@ private void savePlayerData(@NonNull String uniqueID)
                 }
             }
 
-            this.playersDatabase.saveObject(cachedData);
+            this.playersDatabase.saveObjectAsync(cachedData);
         }
     }