Undefined index: source_configuration breaks AJAX

Problem/Motivation

When creating a new Media type entity, the form validation and submit functions use if ($form['source_dependent']['source_configuration']) { to check for the existence of the enclosed value.

This causes an Undefined index error which prevents AJAX from loading the source dependent configuration forms.

Proposed resolution

Use if (array_key_exists('source_configuration', $form['source_dependent'])) { instead.

Remaining tasks

Review

User interface changes

None

API changes

None

Data model changes

None