Magento – override vendor\magento\module-email\Model\AbstractTemplate.php

I want to override method setForcedArea() because I'm using Magento 2.2.4 & whenever I tried to save Content->Configration->Design Configuration->Edit

I'm getting following error:-

Something went wrong while saving this configuration: Area is already set

Screenshot:-

http://nimb.ws/kV93zH

When i R&D then found that it's M2.2.4 issue so I've to modify this method.

Here is the Module :-

Step1:- app/code/Allin/Coreissueresolve/etc/registration.php =>

<?php

\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::MODULE,
    'Allin_Coreissueresolve',
    __DIR__
);

Step2:- app/code/Allin/Coreissueresolve/etc/module.xml =>

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/Module/etc/module.xsd">
    <module name="Allin_Coreissueresolve" setup_version="1.0.0"></module>
        <sequence>
            <module name="Magento_Backend"/>
             <module name="Magento_Sales"/>
            <module name="Magento_Quote"/>
            <module name="Magento_Checkout"/>
        </sequence>
</config>

Step3:- app/code/Allin/Coreissueresolve/etc/di.xml =>

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
<preference for="Magento\Email\Model\AbstractTemplate" type="Allin\Coreissueresolve\Model\Magento\Email\AbstractTemplate" />
</config>

Step4:- app/code/Allin/Coreissueresolve/Model/MagentoEmail/AbstractTemplate.php =>

<?php

namespace Allin\Coreissueresolve\Model\Magento\Email;


abstract class AbstractTemplate extends \Magento\Email\Model\AbstractTemplate
{
     public function setForcedArea($templateId)
    {
        echo "Hello123" ; exit;
    }

}

After that i've run php/bin/magento setup:upgrade & also cache:flush.

But app/code/Allin/Coreissueresolve/Model/MagentoEmail/AbstractTemplate.php => setForcedArea() is not calling.

While vendor\magento\module-email\Model\AbstractTemplate.php => setForcedArea() is calling everytime.