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 class ChallengesManager
         }
 
         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 class ChallengesManager
         //this.levelDatabase = new Database<>(addon, ChallengeLevel.class);
         //this.playersDatabase = new Database<>(addon, ChallengesPlayerData.class);
 
-        loadAndValidate();
+        this.loadAndValidate();
     }
 
 
@@ -263,6 +267,18 @@ public class ChallengesManager
             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 class ChallengesManager
             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 @@ public class ChallengesManager
         {
             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 @@ public class ChallengesManager
      */
     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 class ChallengesManager
      */
     private void saveChallenges()
     {
-        this.challengeCacheData.values().forEach(this.challengeDatabase::saveObject);
+        this.challengeCacheData.values().forEach(this::saveChallenge);
     }
 
 
@@ -845,7 +873,7 @@ public class ChallengesManager
      */
     public void saveChallenge(Challenge challenge)
     {
-        this.challengeDatabase.saveObject(challenge);
+        this.challengeDatabase.saveObjectAsync(challenge);
     }
 
 
@@ -854,7 +882,7 @@ public class ChallengesManager
      */
     private void saveLevels()
     {
-        this.levelCacheData.values().forEach(this.levelDatabase::saveObject);
+        this.levelCacheData.values().forEach(this::saveLevel);
     }
 
 
@@ -864,7 +892,7 @@ public class ChallengesManager
      */
     public void saveLevel(ChallengeLevel level)
     {
-        this.levelDatabase.saveObject(level);
+        this.levelDatabase.saveObjectAsync(level);
     }
 
 
@@ -873,7 +901,7 @@ public class ChallengesManager
      */
     private void savePlayersData()
     {
-        this.playerCacheData.values().forEach(this.playersDatabase::saveObject);
+        this.playerCacheData.values().forEach(this.playersDatabase::saveObjectAsync);
     }
 
 
@@ -902,7 +930,7 @@ public class ChallengesManager
                 }
             }
 
-            this.playersDatabase.saveObject(cachedData);
+            this.playersDatabase.saveObjectAsync(cachedData);
         }
     }