Undefined array key "provider" warning is displayed when an arbitrary module is installed

Problem/Motivation

A warning saying Undefined array key "provider" is displayed when an arbitrary Drupal module is installed. This happens in Elasticsearch Helper module is installed prior to the arbitrary Drupal module.

Steps to reproduce

1. Install Elasticsearch Helper.
2. Have an index plugin enabled which does not provide the "provider" key in the index plugin definition.
3. Install an arbitrary Drupal module.
4. See that the warning Undefined array key "provider" is displayed in the console.