Initializes a plugin and stores the result in $this->pluginInstances.
string $instance_id: The ID of the plugin instance to initialize.
Overrides PluginBag::initializePlugin
protected function initializePlugin($instance_id) {
if (isset($this->pluginInstances[$instance_id])) {
return;
}
$this->pluginInstances[$instance_id] = $this->manager
->createInstance($instance_id, $this->configuration);
}