FATAL ERROR The "field_item:iframe" plugin does not exist

i have created a field of type iframe
then changed the widget
then try and create a new field for that content type and
crash:

Uncaught PHP Exception Drupal\Component\Plugin\Exception\PluginNotFoundException: "The "field_item:iframe" plugin does not exist." at /var/aegir/platforms/drupal8-dev/core/lib/Drupal/Component/Plugin/Discovery/DiscoveryTrait.php line 52"
request: "GET /admin/structure/types/manage/article/fields/node.article.field_type_video/storage?destinations[0][route_name]=entity.field_config.node_field_edit_form&destinations[0][route_parameters][field_config]=node.article.field_type_video&destinations[0][route_parameters][node_type]=article&destinations[1][route_name]=entity.node.field_ui_fields&destinations[1][route_parameters][field_config]=node.article.field_type_video&destinations[1][route_parameters][node_type]=article&destinations[2]=/admin/structure/types/manage/article/fields/add-field

that leads to many error in core/field because the class is null:

PHP Fatal error: Class name must be a valid object or a string in /var/aegir/platforms/drupal8-dev/core/lib/Drupal/Core/Field/FieldConfigStorageBase.php on line 26

PHP Fatal error: Class name must be a valid object or a string in /var/aegir/platforms/drupal8-dev/core/modules/field/src/FieldStorageConfigStorage.php on line 155