Call to a member function getThirdPartySetting() on null in block_class_preprocess_block()

When using some other contrib modules I found the following error in some pages causing WSOD:

The website encountered an unexpected error. Please try again later.
Error: Call to a member function getThirdPartySetting() on null in block_class_preprocess_block() (line 38 of modules/contrib/block_class/block_class.module).

Here is the code of that particular function:

/**
 * Implements hook_preprocess_HOOK().
 */
function block_class_preprocess_block(&$variables) {
  // Blocks coming from page manager widget does not have id.
  if (!empty($variables['elements']['#id'])) {
    $block = Block::load($variables['elements']['#id']);
    if ($classes = $block->getThirdPartySetting('block_class', 'classes')) { // line 38
      $variables['attributes']['class'][] = $classes;
    }
  }
}

It looks that the $block variable is null.