Make sure the target is online.

This ensures that the McMMOPlayer object *should* be valid.

src/main/java/com/gmail/nossr50/util/skills/CombatUtils.java

@@ -452,7 +452,7 @@ public final class CombatUtils {
 
             Player defender = (Player) target;
 
-            if (System.currentTimeMillis() >= UserManager.getPlayer(defender).getRespawnATS() + 5) {
+            if (defender.isOnline() && System.currentTimeMillis() >= UserManager.getPlayer(defender).getRespawnATS() + 5) {
                 baseXP = 20 * Config.getInstance().getPlayerVersusPlayerXP();
             }
         }