common/src/main/java/org/samo_lego/simpleauth/mixin/MixinSlot.java

@@ -16,19 +16,20 @@ public abstract class MixinSlot {
     // Denying item moving etc.
     @Inject(method = "canTakeItems(Lnet/minecraft/entity/player/PlayerEntity;)Z", at = @At(value = "HEAD"), cancellable = true)
     private void canTakeItems(PlayerEntity playerEntity, CallbackInfoReturnable<Boolean> cir) {
-        ServerPlayerEntity player = (ServerPlayerEntity) playerEntity;
-        ActionResult result = AuthEventHandler.onTakeItem(player);
+        if (playerEntity instanceof ServerPlayerEntity player) {
+            ActionResult result = AuthEventHandler.onTakeItem(player);
 
-        if (result == ActionResult.FAIL) {
-            // Canceling the item taking
-            player.networkHandler.sendPacket(
-                    new ScreenHandlerSlotUpdateS2CPacket(
-                            -2,
-                            0,
-                            player.getInventory().selectedSlot,
-                            player.getInventory().getStack(player.getInventory().selectedSlot))
-            );
-            cir.setReturnValue(false);
+            if (result == ActionResult.FAIL) {
+                // Canceling the item taking
+                player.networkHandler.sendPacket(
+                        new ScreenHandlerSlotUpdateS2CPacket(
+                                -2,
+                                0,
+                                player.getInventory().selectedSlot,
+                                player.getInventory().getStack(player.getInventory().selectedSlot))
+                );
+                cir.setReturnValue(false);
+            }
         }
     }
 }

forge/src/main/java/org/samo_lego/simpleauth/SimpleAuthForge.java

@@ -2,9 +2,11 @@ package org.samo_lego.simpleauth;
 
 import com.mojang.brigadier.CommandDispatcher;
 import net.minecraft.server.command.ServerCommandSource;
+import net.minecraftforge.api.distmarker.Dist;
 import net.minecraftforge.common.MinecraftForge;
 import net.minecraftforge.event.RegisterCommandsEvent;
 import net.minecraftforge.eventbus.api.SubscribeEvent;
+import net.minecraftforge.fml.DistExecutor;
 import net.minecraftforge.fml.common.Mod;
 import net.minecraftforge.event.server.ServerStoppedEvent;
 import net.minecraftforge.fml.loading.FMLPaths;
@@ -15,7 +17,7 @@ import org.samo_lego.simpleauth.commands.*;
 public class SimpleAuthForge {
     public SimpleAuthForge() {
         SimpleAuth.init(FMLPaths.GAMEDIR.get());
-        MinecraftForge.EVENT_BUS.register(this);
+        DistExecutor.safeRunWhenOn(Dist.DEDICATED_SERVER, () -> () -> MinecraftForge.EVENT_BUS.register(this));
     }
 
     @SubscribeEvent

forge/src/main/java/org/samo_lego/simpleauth/event/AuthEventHandlerForge.java

@@ -2,6 +2,7 @@ package org.samo_lego.simpleauth.event;
 
 import net.minecraft.entity.player.PlayerEntity;
 import net.minecraft.util.ActionResult;
+import net.minecraftforge.api.distmarker.Dist;
 import net.minecraftforge.event.entity.player.AttackEntityEvent;
 import net.minecraftforge.event.entity.player.PlayerContainerEvent;
 import net.minecraftforge.event.entity.player.PlayerInteractEvent;
@@ -15,7 +16,7 @@ import static org.samo_lego.simpleauth.SimpleAuth.MOD_ID;
  * This class will take care of actions players try to do,
  * and cancel them if they aren't authenticated
  */
-@Mod.EventBusSubscriber(modid = MOD_ID, bus = Mod.EventBusSubscriber.Bus.FORGE)
+@Mod.EventBusSubscriber(value = Dist.DEDICATED_SERVER, modid = MOD_ID, bus = Mod.EventBusSubscriber.Bus.FORGE)
 public class AuthEventHandlerForge {
     @SubscribeEvent(priority = HIGHEST)
     public static void onContainerOpen(PlayerContainerEvent.Open event) {