Now its fixed.

Author: Waterpicker

common/src/main/java/generations/gg/generations/core/generationscore/common/client/screen/mails/MailEditScreen.java

@@ -88,6 +88,7 @@ public MailEditScreen(Player arg, ItemStack arg2, InteractionHand arg3) {
         location = ((MailItem) arg2.getItem()).getType().location;
     }
 
+
     private void setClipboard(String clipboardValue) {
         if (this.minecraft != null) {
             TextFieldHelper.setClipboardContents(this.minecraft, clipboardValue);

common/src/main/java/generations/gg/generations/core/generationscore/common/network/ClientPacketProxy.kt

@@ -31,6 +31,9 @@ class ClientPacketProxy : PacketProxy() {
     }
 
     private fun <T : GenerationsNetworkPacket<T>> createConsumer(handler: ClientNetworkPacketHandler<T>): Consumer<T> {
-        return Consumer { handler.handle(it, Minecraft.getInstance())}
+        return Consumer {
+            var minecraft =Minecraft.getInstance()
+            minecraft.execute { handler.handle(it, minecraft) }
+        }
     }
 }

common/src/main/java/generations/gg/generations/core/generationscore/common/network/packets/S2COpenMailEditScreenPacketHandler.kt

@@ -6,13 +6,14 @@ import generations.gg.generations.core.generationscore.common.client.screen.mail
 import generations.gg.generations.core.generationscore.common.network.ClientNetworkPacketHandler
 import generations.gg.generations.core.generationscore.common.tags.GenerationsItemTags
 import net.minecraft.client.Minecraft
+import kotlin.math.min
 
 object S2COpenMailEditScreenPacketHandler : ClientNetworkPacketHandler<S2COpenMailEditScreenPacket> {
     override fun handle(packet: S2COpenMailEditScreenPacket, minecraft: Minecraft) {
-        val itemStack = Minecraft.getInstance().player!!.getItemInHand(packet.hand)
+        val itemStack = minecraft.player!!.getItemInHand(packet.hand)
         if (itemStack.`is`(GenerationsItemTags.CLOSED_POKEMAIL))
-            Minecraft.getInstance().setScreen(MailViewScreen(WrittenMailAccess(itemStack)))
+            minecraft.setScreen(MailViewScreen(WrittenMailAccess(itemStack)))
         else if (itemStack.`is`(GenerationsItemTags.POKEMAIL))
-            Minecraft.getInstance().setScreen(MailEditScreen(Minecraft.getInstance().player, itemStack, packet.hand))
+            minecraft.setScreen(MailEditScreen(minecraft.player, itemStack, packet.hand))
     }
 }