MessageHandlerOpenGUI.java

     public static class MessageHandlerOpenGUI implements IMessageHandler<MessageOpenGUI, IMessage> {
        @Override
        @SideOnly(Side.SERVER)
        @SuppressWarnings("deprecation")
        public IMessage onMessage(MessageOpenGUI messageIn, MessageContext ctx) {
            final MessageOpenGUI message = messageIn;
            final EntityPlayerMP player = ctx.getServerHandler().player;
            Minecraft.getMinecraft().addScheduledTask(() -> {
                World world = player.world;
                TileEntity te = world.getTileEntity(new BlockPos(message.x, message.y, message.z));
                if (te instanceof ISyncable) {
                    ((ISyncable) te).getSyncPlayerList().add(player);
                    Logger.getGlobal().severe("added player to list");
                }
            });
            return null;
        }

    }