[1.12] How to send changes of book to server?

Hello, i create a custom book (copypast of GuiScreenBook) and i send packet to server, but packet sends after open/close GuiInventory or GuiContainerCreative, not when I'm closing my book.

GuiCustomBook. How to fix it?

private void sendBookToServer() throws IOException
    {
        if (this.bookIsModified && this.bookPages != null)
        {
            while (this.bookPages.tagCount() > 1)
            {
                String s = this.bookPages.getStringTagAt(this.bookPages.tagCount() - 1);

                if (!s.isEmpty())
                {
                    break;
                }

                this.bookPages.removeTag(this.bookPages.tagCount() - 1);
            }

            if (this.bookObj.hasTagCompound())
            {
                NBTTagCompound nbttagcompound = this.bookObj.getTagCompound();
                nbttagcompound.setTag("pages", this.bookPages);
            }
            else
            {
                this.bookObj.setTagInfo("pages", this.bookPages);
            }

            String title = this.bookTitle;

            if (title.equals(TITLE_PLACEHOLDER)) title = "";

            this.bookObj.setTagInfo("title", new NBTTagString(title));

            PacketBuffer packetbuffer = new PacketBuffer(Unpooled.buffer());
            packetbuffer.writeItemStack(this.bookObj);
            this.mc.getConnection().sendPacket(new CPacketCustomPayload("MC|BEdit", packetbuffer));
        }
    }

    @Override
    protected void actionPerformed(GuiButton button) throws IOException
    {
        boolean updateLines = false;
        if (button.enabled)
        {
            switch (button.id)
            {
                case BUTTON_DONE_ID:
                    this.mc.displayGuiScreen(null);
                    this.sendBookToServer();
                    break;
            }
        }
    }