Umm...

17:28:41[ERROR] Could not pass event InventoryCloseEvent to SiliqonMail v1.0
java.lang.ArrayIndexOutOfBoundsException: Index 36 out of bounds for length 36
at java.base/java.util.Arrays$ArrayList.set(Unknown Source) ~[?:?]
at net.minecraft.core.NonNullList.set(NonNullList.java:50) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at org.bukkit.craftbukkit.inventory.CraftInventoryCustom$MinecraftInventory.setItem(CraftInventoryCustom.java:167) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at org.bukkit.craftbukkit.inventory.CraftInventory.setItem(CraftInventory.java:100) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at org.bukkit.craftbukkit.inventory.CraftInventory.clear(CraftInventory.java:445) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at SiliqonMail-1.0.jar/com.siliqon.siliqonmail.gui.menus.SendMailMenu.onClose(SendMailMenu.java:74) ~[SiliqonMail-1.0.jar:?]
at SiliqonMail-1.0.jar/com.siliqon.siliqonmail.gui.GUIManager.handleClose(GUIManager.java:48) ~[SiliqonMail-1.0.jar:?]
at SiliqonMail-1.0.jar/com.siliqon.siliqonmail.listeners.GUIListener.onClose(GUIListener.java:32) ~[SiliqonMail-1.0.jar:?]
at com.destroystokyo.paper.event.executor.asm.generated.GeneratedEventExecutor80.execute(Unknown Source) ~[?:?]
at org.bukkit.plugin.EventExecutor$2.execute(EventExecutor.java:77) ~[paper-mojangapi-1.21.1-R0.1-SNAPSHOT.jar:?]
at co.aikar.timings.TimedEventExecutor.execute(TimedEventExecutor.java:80) ~[paper-mojangapi-1.21.1-R0.1-SNAPSHOT.jar:1.21.1-2314-501e7c5]
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:70) ~[paper-mojangapi-1.21.1-R0.1-SNAPSHOT.jar:?]
at io.papermc.paper.plugin.manager.PaperEventManager.callEvent(PaperEventManager.java:54) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at io.papermc.paper.plugin.manager.PaperPluginManagerImpl.callEvent(PaperPluginManagerImpl.java:131) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:630) ~[paper-mojangapi-1.21.1-R0.1-SNAPSHOT.jar:?]
at org.bukkit.craftbukkit.event.CraftEventFactory.handleInventoryCloseEvent(CraftEventFactory.java:1656) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at net.minecraft.server.network.ServerGamePacketListenerImpl.handleContainerClose(ServerGamePacketListenerImpl.java:3039) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at net.minecraft.server.network.ServerGamePacketListenerImpl.handleContainerClose(ServerGamePacketListenerImpl.java:3032) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at net.minecraft.network.protocol.game.ServerboundContainerClosePacket.handle(ServerboundContainerClosePacket.java:33) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at net.minecraft.network.protocol.game.ServerboundContainerClosePacket.handle(ServerboundContainerClosePacket.java:15) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at net.minecraft.network.protocol.PacketUtils.lambda$ensureRunningOnSameThread$0(PacketUtils.java:56) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at net.minecraft.server.TickTask.run(TickTask.java:18) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at net.minecraft.util.thread.BlockableEventLoop.doRunTask(BlockableEventLoop.java:151) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at net.minecraft.util.thread.ReentrantBlockableEventLoop.doRunTask(ReentrantBlockableEventLoop.java:24) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at net.minecraft.server.MinecraftServer.doRunTask(MinecraftServer.java:1572) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at net.minecraft.server.MinecraftServer.doRunTask(MinecraftServer.java:201) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at net.minecraft.util.thread.BlockableEventLoop.pollTask(BlockableEventLoop.java:125) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at net.minecraft.server.MinecraftServer.pollTaskInternal(MinecraftServer.java:1549) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at net.minecraft.server.MinecraftServer.pollTask(MinecraftServer.java:1542) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at net.minecraft.util.thread.BlockableEventLoop.runAllTasks(BlockableEventLoop.java:114) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at net.minecraft.server.MinecraftServer.tickServer(MinecraftServer.java:1664) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1333) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:333) ~[purpur-1.21.1.jar:1.21.1-2314-501e7c5]
at java.base/java.lang.Thread.run(Unknown Source) ~[?:?]

This plugin is good, but when a player goes to send something to another player, and accidentally presses ESC or turns it off, the things he sent him will disappear and he won't get them back