fix fireball bug on spam

scripts/witch.gd

@@ -58,6 +58,8 @@ func shoot_fireballs():
 	enemies.sort_custom(func(a, b): return global_position.distance_to(a.global_position) < global_position.distance_to(b.global_position))
 	var targets = enemies.slice(0, fireball_max_targets)
 	for enemy in targets:
+		if not is_instance_valid(enemy):
+			continue
 		var dir = global_position.direction_to(enemy.global_position)
 		_face_direction(dir)
 		var fb = fireball.instantiate()