Configured headers not used anymore

I created a MR, just straightforward resetting it to what it was before the change.

That will cause error messages when testing this with PhpStan. There needs to be a way to safely get to those values, especially in PHP 8.1

Can you verify the bug?

Might it be possible to do a isset($header->key) check? This should trigger the magic __isset() method in Drupal\Core\Field\FieldItemBase

How about this fully sanitized method:

  public function getHeader(): array {
    $headers = [];
    /** @var \Drupal\key_value_field\Plugin\Field\FieldType\KeyValueItem $header */
    foreach ($this->get('header') as $header) {
      try {
        $key = $header->get('key')->getValue();
        $value = $header->get('value')->getValue();
        $headers[$key] = $value;
      }
      catch (MissingDataException $e) {
        // We can ignore this.
      }
    }
    return $headers;
  }

For a missing property it throws a throw new \InvalidArgumentException("Property $property_name is unknown."); in web/core/lib/Drupal/Core/TypedData/TypedDataManager.php on my end.

I can't reproduce that, it works just fine here.