Data Migration Error

[mcpacific]

I am migrating a CE 1.9.4.1 site to CE 2.3.5 and have installed the data migration tool. All the integrity checks complete with no errors (including the EAV Step) but I get the following error:

In ErrorHandler.php line 61:

[Exception]
Notice: Undefined offset: 21 in /vendor/magento/data-migration-tool/src/Migration/Step/Eav/Data.php on line 492

Exception trace:
  at /vendor/magento/framework/App/ErrorHandler.php:61
 Magento\Framework\App\ErrorHandler->handler() at /vendor/magento/data-migration-tool/src/Migration/Step/Eav/Data.php:492
 Migration\Step\Eav\Data->migrateCustomEntityAttributes() at /vendor/magento/data-migration-tool/src/Migration/Step/Eav/Data.php:183
 Migration\Step\Eav\Data->perform() at /vendor/magento/data-migration-tool/src/Migration/Mode/AbstractMode.php:82
 Migration\Mode\AbstractMode->runStage() at /vendor/magento/data-migration-tool/src/Migration/Mode/Data.php:124
 Migration\Mode\Data->runData() at /vendor/magento/data-migration-tool/src/Migration/Mode/Data.php:69
 Migration\Mode\Data->run() at /vendor/magento/data-migration-tool/src/Migration/Console/MigrateDataCommand.php:59
 Migration\Console\MigrateDataCommand->execute() at /vendor/symfony/console/Command/Command.php:255
 Symfony\Component\Console\Command\Command->run() at /vendor/magento/framework/Interception/Interceptor.php:58
 Migration\Console\MigrateDataCommand\Interceptor->___callParent() at /vendor/magento/framework/Interception/Interceptor.php:138
 Migration\Console\MigrateDataCommand\Interceptor->Magento\Framework\Interception\{closure}() at /vendor/magento/framework/Interception/Interceptor.php:153
 Migration\Console\MigrateDataCommand\Interceptor->___callPlugins() at /generated/code/Migration/Console/MigrateDataCommand/Interceptor.php:104
 Migration\Console\MigrateDataCommand\Interceptor->run() at /vendor/symfony/console/Application.php:1001

I have search for solutions and nothing seems to work to get me past this error.

[mcpacific · Accepted solution]

I guess I figured this one out on my own. Looking at 

/vendor/magento/data-migration-tool/src/Migration/Step/Eav/Data.php on line 492

I realized the offset '21' was the issue. I found the attribute_group_id matching the offset in the eav_attribute_group table in the M1 database and deleted it.

On subsequent runs, there were two more that had to be removed. After removing all three, problem solved. I hope this helps someone else overcome this error.