Undefined array key "#attached" in Drupal\shs\Plugin\Field\FieldWidget\OptionsShsWidget::afterBuild()

Problem/Motivation

When editing a field settings on a content type that uses shs widget in form display this error is shown (due to default value field):

Error message
Warning: Undefined array key "#attached" in Drupal\shs\Plugin\Field\FieldWidget\OptionsShsWidget::afterBuild() (line 270 of modules/contrib/shs/src/Plugin/Field/FieldWidget/OptionsShsWidget.php).

Drupal\shs\Plugin\Field\FieldWidget\OptionsShsWidget::afterBuild(Array, Object)
call_user_func_array(Array, Array) (Line: 1083)

Proposed resolution

We could use null coalescing operator \Drupal\shs\Plugin\Field\FieldWidget\OptionsShsWidget::afterBuild instead of ternary operator.

    $element['#attached'] = $element['#attached'] ?? [];
    $element['#attached'] = array_merge($element['#attached'], [
      'drupalSettings' => [
        'shs' => [
          $element_key => $element['#shs'],
        ],
      ],
    ]);