When you use CustomForm to create a UI and submit any data in a UI the FormValidation error would occur and send the output in console (it's down below) but would NOT crash the client.
This issue has been reported before there is no need to create another issue, we will find a solution to this problem.
2021-12-06 [07:38:41.678] [Server thread/CRITICAL]: pocketmine\form\FormValidationException: "Expected an array response with the size 0, got 2" (EXCEPTION) in "plugins/StatsX (1).phar/src/Vecnavium/StatsX/Forms/CustomForm" at line 30
--- Stack trace ---
#0 plugins/StatsX (1).phar/src/Vecnavium/StatsX/Forms/Form(43): Vecnavium\StatsX\Forms\CustomForm->processData(array[2])
#1 pmsrc/src/player/Player(1908): Vecnavium\StatsX\Forms\Form->handleResponse(object pocketmine\player\Player#25071, array[2])
#2 pmsrc/src/network/mcpe/handler/InGamePacketHandler(774): pocketmine\player\Player->onFormSubmit(integer 0, array[2])
#3 pmsrc/vendor/pocketmine/bedrock-protocol/src/ModalFormResponsePacket(55): pocketmine\network\mcpe\handler\InGamePacketHandler->handleModalFormResponse(object pocketmine\network\mcpe\protocol\ModalFormResponsePacket#88830)
#4 pmsrc/src/network/mcpe/NetworkSession(395): pocketmine\network\mcpe\protocol\ModalFormResponsePacket->handle(object pocketmine\network\mcpe\handler\InGamePacketHandler#93410)
#5 pmsrc/src/network/mcpe/NetworkSession(358): pocketmine\network\mcpe\NetworkSession->handleDataPacket(object pocketmine\network\mcpe\protocol\ModalFormResponsePacket#88830, string[22] e..[null,"Vecnavium"].)
#6 pmsrc/src/network/mcpe/raklib/RakLibInterface(192): pocketmine\network\mcpe\NetworkSession->handleEncoded(string[25] .Ke...+...Q.KM.K,.,.U....)
#7 pmsrc/vendor/pocketmine/raklib-ipc/src/RakLibToUserThreadMessageReceiver(42): pocketmine\network\mcpe\raklib\RakLibInterface->onPacketReceive(integer 0, string[34] .G..........= ,%&..Oj...f'.....}+m)
#8 pmsrc/src/network/mcpe/raklib/RakLibInterface(122): raklib\server\ipc\RakLibToUserThreadMessageReceiver->handle(object pocketmine\network\mcpe\raklib\RakLibInterface#23437)
#9 pmsrc/vendor/pocketmine/snooze/src/SleeperHandler(123): pocketmine\network\mcpe\raklib\RakLibInterface->pocketmine\network\mcpe\raklib\{closure}()
#10 pmsrc/vendor/pocketmine/snooze/src/SleeperHandler(82): pocketmine\snooze\SleeperHandler->processNotifications()
#11 pmsrc/src/Server(1645): pocketmine\snooze\SleeperHandler->sleepUntil(double 1638808721.6146)
#12 pmsrc/src/Server(1034): pocketmine\Server->tickProcessor()
#13 pmsrc/src/PocketMine(301): pocketmine\Server->__construct(object BaseClassLoader#2, object pocketmine\utils\MainLogger#3, string[46] A:\PocketMine-MP\PM4 (Testing plugins server)\, string[54] A:\PocketMine-MP\PM4 (Testing plugins server)\plugins\)
#14 pmsrc/src/PocketMine(324): pocketmine\server()
#15 pmsrc(11): require(string[90] phar://A:/PocketMine-MP/PM4 (Testing plugins server)/PocketMine-MP.phar/src/Pock)
--- End of exception information --- ```