RemoveSectionForm assumes there is a label

Problem/Motivation

In #3073872: Allow for Layout Builder sections to be given administrative labels we added administrative labels, which added a new default configuration 'label' to the LayoutDefault class. However, layouts may choose to use a different class that might not have a label configuration, which results in an notice when trying to remove a section.

Notice: Undefined index: label in Drupal\layout_builder\Form\RemoveSectionForm->getQuestion() (line 28 of /var/www/html/web/core/modules/layout_builder/src/Form/RemoveSectionForm.php)

  public function getQuestion() {
    $configuration = $this->sectionStorage->getSection($this->delta)->getLayoutSettings();
    if ($configuration['label']) {
      return $this->t('Are you sure you want to remove @section?', ['@section' => $configuration['label']]);
    }

We're already being defensive in other places, such as in Element/LayoutBuilder.php:

$section_label = !empty($layout_settings['label']) ? $layout_settings['label'] : $this->t('Section @section', ['@section' => $delta + 1]);

So we should be here as well!

Steps to reproduce

Proposed resolution

Ensure we check that a label key actually exists in the configuration in RemoveSectionForm.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet