Fix NPE when opening ChallengesGUI (#116).

Add code that removes challenges level from all its challenges, if level is deleted.
Add protection code that shows admins that something is not correct if challenge level is not in database.

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

@@ -1462,6 +1462,20 @@ public class ChallengesManager
         if (this.levelCacheData.containsKey(challengeLevel.getUniqueId()))
         {
             this.levelCacheData.remove(challengeLevel.getUniqueId());
+
+            // Remove challenge level from challenges object.
+            if (!challengeLevel.getChallenges().isEmpty())
+            {
+                challengeLevel.getChallenges().forEach(challengeID -> {
+                    Challenge challenge = this.getChallenge(challengeID);
+
+                    if (challenge != null)
+                    {
+                        challenge.setLevel(ChallengesManager.FREE);
+                    }
+                });
+            }
+
             this.levelDatabase.deleteObject(challengeLevel);
         }
     }

src/main/java/world/bentobox/challenges/panel/CommonGUI.java

@@ -298,8 +298,19 @@ public abstract class CommonGUI
 			{
 				case 'l':
 				{
-					result.add(this.user.getTranslation("challenges.gui.challenge-description.level",
-						"[level]", manager.getLevel(challenge).getFriendlyName()));
+					ChallengeLevel level = manager.getLevel(challenge);
+
+					if (level == null)
+					{
+						result.add(this.user.getTranslation("challenges.errors.missing-level",
+							"[level]", challenge.getLevel()));
+					}
+					else
+					{
+						result.add(this.user.getTranslation("challenges.gui.challenge-description.level",
+							"[level]", level.getFriendlyName()));
+					}
+
 					break;
 				}
 				case 's':

src/main/java/world/bentobox/challenges/tasks/TryToComplete.java

@@ -359,12 +359,13 @@ public class TryToComplete
         // Mark as complete
         this.manager.setChallengeComplete(this.user, this.world, this.challenge, result.getFactor());
 
-        // Check level completion.
-        if (!result.wasCompleted())
+        // Check level completion for non-free challenges
+        if (!result.wasCompleted() &&
+            !this.challenge.getLevel().equals(ChallengesManager.FREE))
         {
             ChallengeLevel level = this.manager.getLevel(this.challenge);
 
-            if (!this.manager.isLevelCompleted(this.user, this.world, level))
+            if (level != null && !this.manager.isLevelCompleted(this.user, this.world, level))
             {
                 if (this.manager.validateLevelCompletion(this.user, this.world, level))
                 {

src/main/resources/locales/en-US.yml

@@ -364,6 +364,7 @@ challenges:
         defaults-file-exist: '&cdefault.json already exists. Use overwrite mode to replace it!'
         defaults-file-error: '&cThere was an error while creating default.json file! Check console!'
         no-challenges: '&cChallenges are not implemented in current world!'
+        missing-level: '&cChallenge Level [level] is not defined in database. It may case some errors!'
 protection:
     flags:
         CHALLENGES_ISLAND_PROTECTION: