PluginNotFoundException: "The "iframe" plugin does not exist." at core/lib/Drupal/Component/P‌​lugin/Discovery/Disc‌​overyTrait.php line 52" how to solve such error: 

Uncaught PHP Exception Drupal\Component\Plugin\Exception\PluginNotFoundException: "The "iframe" plugin does not exist." at /var/aegir/platforms/drupal8-dev/core/lib/Drupal/Component/P‌​lugin/Discovery/Disc‌​overyTrait.php line 52" while reading response header from upstream, request: "GET /admin/config/content/formats/manage/basic_html HTTP/1.1"


The plugin 'iframe' have been existing since i found it in the enabled filter list 

  http://drupal.stackexchange.com/questions/218781/create-a-new-text-format-filter


then i believe when i have clear the cache at some stage it came up with this exception  when trying to acces again to 

        admin/config/content/formats/manage/basic_html


where i have initially add this plugin filter

The exception come up when i am trying to uninstall the module so it is problematic

it seems that this error is reccurent when i googleized it but i did not found where the problem lies
removing manually the module and deleting the %iframe% entry in config table should do 

it happens when  there's no definitition of the plugin

   /*protected function doGetDefinition(array $definitions, $plugin_id, $exception_on_invalid) {
// Avoid using a ternary that would create a copy of the array.
if (isset($definitions[$plugin_id])) {
    return $definitions[$plugin_id];
}
elseif (!$exception_on_invalid) {
    return NULL;
}

throw new PluginNotFoundException($plugin_id, sprintf('The "%s" plugin does not exist.', $plugin_id));


so what i understand is my custom module dont have a valid plugin manager !?

https://www.drupal.org/docs/8/api/plugin-api/d8-plugin-discovery