Fix null-pointer issue when trying to get first element of empty list.

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

@@ -435,7 +435,7 @@ private void savePlayerData(@NonNull String uniqueID)
 
                 Iterator<LogEntry> entryIterator = cachedData.getHistory().iterator();
 
-                while (this.shouldBeRemoved(entryIterator.next(), survivalTime))
+                while (entryIterator.hasNext() && this.shouldBeRemoved(entryIterator.next(), survivalTime))
                 {
                     entryIterator.remove();
                 }