Slappers are invisible

[Bruth78]

Hello.
So whenever I try to add a Slapper nothing happens.
When I try to edit it’s id, the slapper doesn’t exist and whenever I restart the server this happens: Fatal error: Uncaught Error: Call to undefined method stdClass::isClosed() in phar:///home/container/PocketMine-MP.phar/src/pocketmine/level/Position.php:98
Stack trace:
#0 phar:///home/container/PocketMine-MP.phar/src/pocketmine/entity/Entity.php(2090): pocketmine\level\Position->isValid()
#1 phar:///home/container/PocketMine-MP.phar/src/pocketmine/entity/Living.php(932): pocketmine\entity\Entity->close()
#2 phar:///home/container/PocketMine-MP.phar/src/pocketmine/entity/Human.php(898): pocketmine\entity\Living->close()
#3 phar:///home/container/PocketMine-MP.phar/src/pocketmine/entity/Entity.php(2180): pocketmine\entity\Human->close()
#4 [internal function]: pocketmine\entity\Entity->__destruct()
#5 {main}
thrown in phar:///home/container/PocketMine-MP.phar/src/pocketmine/level/Position.php on line 98