Changes to fit new API for database transition. (#114)

Config files must implement the ConfigObject interface now instead of
DataObject. They no longer need to specify uniqueId.

src/main/java/world/bentobox/challenges/database/object/Challenge.java

@@ -311,7 +311,7 @@ public class Challenge implements DataObject
      */
     public ItemStack getIcon()
     {
-        return icon.clone();
+        return icon !=null ? icon.clone() : new ItemStack(Material.PAPER);
     }