Fix for issue #21510: Can't access backend indexers page after creating a custom index

[ccasciotti]

Description (*)

This PR is a fix for issue #21510.

Fixed Issues (if relevant)

1. Can't access backend indexers page after creating a custom index #21510: Can't access backend indexers page after creating a custom index

Manual testing scenarios (*)

1. Implementation of a custom index as the guide: https://devdocs.magento.com/guides/v2.2/extension-dev-guide/indexing-custom.html
2. Login to backend
3. Go to System > Index Management

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @ccasciotti. Thank you for your contribution
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[magento-engcom-team]

Hi @VladimirZaets, thank you for the review.
ENGCOM-4440 has been created to process this Pull Request

[magento-engcom-team]

Hi @sidolov, thank you for the review.
ENGCOM-4440 has been created to process this Pull Request

[soleksii]

Hi @ccasciotti! I cannot reproduce the issue #21510 on 2.3-develop instance.
Please, re-check.
Manual testing scenario:

1. Implementation of a custom index as the guide: https://devdocs.magento.com/guides/v2.2/extension-dev-guide/indexing-custom.html
2. Login to backend
3. Go to System > Index Management
   ✔️ A successful response with the list of indexers
   

[ccasciotti]

Hi @stoleksiy,
I can confirm the issue with the above steps, I can also add that you don't have to run a reindex nor the setup:upgrade command.
The ideal scenario would be to add a new custom index to an existing module following the steps in the official guide.

[soleksii]

✔️ QA Passed

Manual testing scenario:

1. Implementation of a custom index as the guide: https://devdocs.magento.com/guides/v2.2/extension-dev-guide/indexing-custom.html;
2. Login to backend;
3. Go to System > Index Management

Result:

[ghost]

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