Undefined array key "#markup" in Drupal\config_translation\FormElement\ListElement->getGroupTitle()

Problem/Motivation

A Warning is thrown when the translation is enabled for a config form and one of the form elements key contains the string "title" or "label"

Steps to reproduce

• Create a custom config form with the following element :
  

   $form['section_title'] = [
          '#type' => 'text_format',
          '#title' => 'Section Title',
          '#allowed_formats' => ['title_html'],
          '#format' => 'title_html',
          '#description' => $this->t('Enter the title here.'),
          '#description_display' => 'before',
          '#default_value' => $config->get("section_title")['value'] ?? '',
          '#required' => TRUE,
        ];

• Add Schema file for the same
  

  mapping:
              section_title:
                type: text_format
                label: 'Section Title'

• Now try to submit the translation form you'll see the warning above the form elements

Proposed resolution

Add a check if "#markup" exists in the source of the element before assigning to title.