generate field long name by shop method, assert return type

Modules/Application/Model/D3ManufacturerLongDesc.php

@@ -47,7 +47,7 @@ class D3ManufacturerLongDesc extends D3ManufacturerLongDesc_parent
      */
     public function d3GetLongdesc(): Field
     {
-        $longName = $this->getCoreTableName().'__'.strtolower(Constants::D3_OXMANUFACTURERS_LONGDESC_FIELDNAME);
-        return $this->$longName;
+        $longName = $this->_getFieldLongName(Constants::D3_OXMANUFACTURERS_LONGDESC_FIELDNAME);
+        return $this->$longName ?: oxNew(Field::class);
     }
 }