Multiple 'Warning: Undefined array key' errors in Canvas editor

Problem/Motivation

This module generates multiple php errors when editing a canvas page

Steps to reproduce

- Add and enable this module, make sure you have a languages dropdown block, for example on your page heading navigation
- Use Drupal Canvas, create or edit a canvas page
- Visit /admin/reports/dblog en notice multiple php errors from this module

Proposed resolution

It all comes from missing or empty array values in languages_dropdown/src/Plugin/Block/LangDropBootstrapBlock.php. In the blockForm function, fallbacks are always provided in case the array keys do not exist, like:

    if (isset($config['languages_dropdown_bootstrap']) && !empty($config['languages_dropdown_bootstrap'])) {
      $settings = $config['languages_dropdown_bootstrap'];
    }
    else {
      $settings = [];
    }

However, in other functions, there are no checks so these result in php errors. For example in the function build() :
$settings = $this->configuration['languages_dropdown_bootstrap'];
This throws an error when editing a canvas page, but could be fixed like:
$settings = $this->configuration['languages_dropdown_bootstrap'] ?? [];