Handle null player in resetFlagCarrier method

Added a null check for the flagCarrier player before clearing active potion effects to prevent potential NullPointerExceptions. This ensures stable behavior when resetting the flag carrier.

src/main/java/com/alttd/ctf/flag/Flag.java

@@ -357,7 +357,9 @@ public class Flag implements Runnable {
 
     public void resetFlagCarrier() {
         final Player player = flagCarrier;
-        Bukkit.getScheduler().runTask(main, player::clearActivePotionEffects);
+        if (player != null) {
+            Bukkit.getScheduler().runTask(main, player::clearActivePotionEffects);
+        }
         flagCarrier = null;
         winningTeam = null;
         particleTrail.clear();