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.
To help us process this issue please make sure that you provided the following information:

• Summary of the issue
• Information on your environment
• Steps to reproduce
• Expected and actual results

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:

@magento-engcom-team give me 2.3-develop instance - upcoming 2.3.x release

For more details, please, review the Magento Contributor Assistant documentation.

@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.

[magento-engcom-team]

Hi @ccasciotti. Thank you for your report.
The issue has been fixed in #21575 by @ccasciotti in 2.3-develop branch
Related commit(s):

• 318425fafd4d92dbabcf89d332dedf9f27978f63
• 486e21c19a6678d3a1b6b0e259408fb1d205bceb
• 409e00e434f156501e1f1ae6d734d74ae82d70f3
• 0acd6b6a0982941c11f1edfc10afd7896138222e

The fix will be available with the upcoming 2.3.2 release.

[magento-engcom-team]

Hi @ccasciotti. Thank you for your report.
The issue has been fixed in #21576 by @ccasciotti in 2.2-develop branch
Related commit(s):

• 2c956f661caf1a10b1b6e6afea185fd473313fd3
• d5ed9e89b41c6d407869d4cf00f8293fb5f6e41f

The fix will be available with the upcoming 2.2.9 release.