I used the version of this plugin from master as of the time making this report, stack is below.
To reproduce: /snpc spawn Human "test"
[18:11:12] [Server thread/CRITICAL]: #0 plugins/SimpleNPC-master/src/brokiem/snpc/entity/CustomHuman(16): pocketmine\entity\Human->__construct(object pocketmine\level\Level, object pocketmine\nbt\tag\CompoundTag)
[18:11:12] [Server thread/CRITICAL]: #1 plugins/SimpleNPC-master/src/brokiem/snpc/manager/NPCManager(83): brokiem\snpc\entity\CustomHuman->__construct(object pocketmine\level\Level, object pocketmine\nbt\tag\CompoundTag)
[18:11:12] [Server thread/CRITICAL]: #2 plugins/SimpleNPC-master/src/brokiem/snpc/manager/NPCManager(60): brokiem\snpc\manager\NPCManager::createEntity(string[10] human_snpc, object pocketmine\level\Level, object pocketmine\nbt\tag\CompoundTag)
[18:11:12] [Server thread/CRITICAL]: #3 plugins/SimpleNPC-master/src/brokiem/snpc/commands/Commands(206): brokiem\snpc\manager\NPCManager::createNPC(string[10] human_snpc, object pocketmine\Player, string[4] test)
[18:11:12] [Server thread/CRITICAL]: #4 pmsrc/src/pocketmine/command/SimpleCommandMap(248): brokiem\snpc\commands\Commands->execute(object pocketmine\Player, string[4] snpc, array[3])
[18:11:12] [Server thread/CRITICAL]: #5 pmsrc/src/pocketmine/Server(1809): pocketmine\command\SimpleCommandMap->dispatch(object pocketmine\Player, string[23] snpc spawn Human "test")
[18:11:12] [Server thread/CRITICAL]: #6 pmsrc/src/pocketmine/Player(2351): pocketmine\Server->dispatchCommand(object pocketmine\Player, string[23] snpc spawn Human "test")
[18:11:12] [Server thread/CRITICAL]: #7 pmsrc/src/pocketmine/network/mcpe/PlayerNetworkSessionAdapter(239): pocketmine\Player->chat(string[24] /snpc spawn Human "test")
[18:11:12] [Server thread/CRITICAL]: #8 pmsrc/src/pocketmine/network/mcpe/protocol/CommandRequestPacket(54): pocketmine\network\mcpe\PlayerNetworkSessionAdapter->handleCommandRequest(object pocketmine\network\mcpe\protocol\CommandRequestPacket)
[18:11:12] [Server thread/CRITICAL]: #9 pmsrc/src/pocketmine/network/mcpe/PlayerNetworkSessionAdapter(109): pocketmine\network\mcpe\protocol\CommandRequestPacket->handle(object pocketmine\network\mcpe\PlayerNetworkSessionAdapter)
[18:11:12] [Server thread/CRITICAL]: #10 pmsrc/src/pocketmine/network/mcpe/protocol/BatchPacket(130): pocketmine\network\mcpe\PlayerNetworkSessionAdapter->handleDataPacket(object pocketmine\network\mcpe\protocol\CommandRequestPacket)
[18:11:12] [Server thread/CRITICAL]: #11 pmsrc/src/pocketmine/network/mcpe/PlayerNetworkSessionAdapter(109): pocketmine\network\mcpe\protocol\BatchPacket->handle(object pocketmine\network\mcpe\PlayerNetworkSessionAdapter)
[18:11:12] [Server thread/CRITICAL]: #12 pmsrc/src/pocketmine/Player(3311): pocketmine\network\mcpe\PlayerNetworkSessionAdapter->handleDataPacket(object pocketmine\network\mcpe\protocol\BatchPacket)
[18:11:12] [Server thread/CRITICAL]: #13 pmsrc/src/pocketmine/network/mcpe/RakLibInterface(169): pocketmine\Player->handleDataPacket(object pocketmine\network\mcpe\protocol\BatchPacket)
[18:11:12] [Server thread/CRITICAL]: #14 pmsrc/vendor/pocketmine/raklib/src/server/ServerHandler(95): pocketmine\network\mcpe\RakLibInterface->handleEncapsulated(string[15] 127.0.0.1 60529, object raklib\protocol\EncapsulatedPacket, integer 0)
[18:11:12] [Server thread/CRITICAL]: #15 pmsrc/src/pocketmine/network/mcpe/RakLibInterface(109): raklib\server\ServerHandler->handlePacket()
[18:11:12] [Server thread/CRITICAL]: #16 pmsrc/src/pocketmine/network/mcpe/RakLibInterface(99): pocketmine\network\mcpe\RakLibInterface->process()
[18:11:12] [Server thread/CRITICAL]: #17 pmsrc/vendor/pocketmine/snooze/src/SleeperHandler(113): pocketmine\network\mcpe\RakLibInterface->pocketmine\network\mcpe\{closure}()
[18:11:12] [Server thread/CRITICAL]: #18 pmsrc/vendor/pocketmine/snooze/src/SleeperHandler(75): pocketmine\snooze\SleeperHandler->processNotifications()
[18:11:12] [Server thread/CRITICAL]: #19 pmsrc/src/pocketmine/Server(2155): pocketmine\snooze\SleeperHandler->sleepUntil(double 1617923472.9187)
[18:11:12] [Server thread/CRITICAL]: #20 pmsrc/src/pocketmine/Server(1992): pocketmine\Server->tickProcessor()
[18:11:12] [Server thread/CRITICAL]: #21 pmsrc/src/pocketmine/Server(1586): pocketmine\Server->start()
[18:11:12] [Server thread/CRITICAL]: #22 pmsrc/src/pocketmine/PocketMine(291): pocketmine\Server->__construct(object BaseClassLoader, object pocketmine\utils\MainLogger, string[23] G:\Backups\Test Server\, string[31] G:\Backups\Test Server\plugins\)
[18:11:12] [Server thread/CRITICAL]: #23 pmsrc/src/pocketmine/PocketMine(321): pocketmine\server()
[18:11:12] [Server thread/CRITICAL]: #24 pmsrc(11): require(string[78] phar://G:/Backups/Test Server/PocketMine-MP.phar/src/pocketmine/PocketMine.php)```