Can't access backend indexers page after creating a custom index

[ccasciotti]

After complete all the steps to create a new custom index as described in the official devdoc guide (https://devdocs.magento.com/guides/v2.2/extension-dev-guide/indexing-custom.html)
if you go on admin's indexer list page a fatal error is thrown due to a partially wrong implementation of Magento\Indexer\Model\Indexer::getLatestUpdated method called by Magento\Indexer\Model\Indexer\DependencyDecorator::getLatestUpdated.
The result of the DependencyDecorator::getLatestUpdated must be a string, but the Indexer::getLatestUpdated method returns null the very first time you access that backend page because no view is yet present.

Preconditions (*)

1. Magento 2.2.7 (both community or commerce)

Steps to reproduce (*)

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

Expected result (*)

1. A successful response with the list of indexers
   

Actual result (*)

1. A fatal error with this exception:
   PHP Fatal error: Uncaught TypeError: Return value of Magento\\Indexer\\Model\\Indexer\\DependencyDecorator::getLatestUpdated() must be of the type string, null returned in <magento_root>/vendor/magento/module-indexer/Model/Indexer/DependencyDecorator.php:240

[magento-engcom-team]

Hi @ccasciotti. Thank you for your report.
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Please make sure that the issue is reproducible on the vanilla Magento instance following Steps to reproduce. To deploy vanilla Magento instance on our environment, please, add a comment to the issue:

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@ccasciotti do you confirm that you was able to reproduce the issue on vanilla Magento instance following steps to reproduce?

• yes
• no

[magento-engcom-team]

Hi @engcom-backlog-nazar. Thank you for working on this issue.
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[magento-engcom-team]

✅ Confirmed by @engcom-backlog-nazar
Thank you for verifying the issue. Based on the provided information internal tickets MAGETWO-98528, MAGETWO-98529 were created

Issue Available: @engcom-backlog-nazar, You will be automatically unassigned. Contributors/Maintainers can claim this issue to continue. To reclaim and continue work, reassign the ticket to yourself.