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