Error saving pane

Problem/Motivation

When creating my first pane at saving I get

An error has occurred unexpectedly. Try again later.

Drupal\Component\Plugin\Exception\PluginNotFoundException: The "" plugin does not exist. Valid plugin IDs for Drupal\media_library_extend\Plugin\MediaLibrarySourceManager are: lorem_picsum, configurable_lorem_picsum, youtube_channel in Drupal\Core\Plugin\DefaultPluginManager->doGetDefinition() (line 53 of core/lib/Drupal/Component/Plugin/Discovery/DiscoveryTrait.php).
Drupal\Core\Plugin\DefaultPluginManager->getDefinition(NULL) (Line: 16)
Drupal\Core\Plugin\Factory\ContainerFactory->createInstance(NULL, Array) (Line: 83)
Drupal\Component\Plugin\PluginManagerBase->createInstance(NULL, Array) (Line: 118)
Drupal\media_library_extend\Entity\MediaLibraryPane->getPlugin() (Line: 64)
Drupal\media_library_extend\MediaLibraryPaneListBuilder->buildRow(Object) (Line: 235)
Drupal\Core\Entity\EntityListBuilder->render() (Line: 23)
Drupal\Core\Entity\Controller\EntityListController->listing('media_library_pane')
call_user_func_array(Array, Array) (Line: 123)

Steps to reproduce

Create pane and ignore the required source plugin since it provides an empty list.

Proposed resolution

Error when required field is not used (and a little help how to get a source plugin)