Q:

CakePHP3 "Helper not found" error in plugin

I have created a helper "SideNavbarHelper.php" in {appname}/plugins/UsefulHelpers/src/View/Helper, 
and I am trying to set helper in beforeRender in AppController, but I got following error. 

Error: UsefulHelpers. SideNavbarHelper could not be found. 

beforeRender in AppController.php
public function beforeRender(Event $event) {
    parent::beforeRender($event);
    $this->helpers["Navbar"]  =  ["className"=>"UsefulHelpers. SideNavbar"];
}

SideNavbarHelper.php
namespace UsefulHelpers\View\Helper;
use Cake\View\Helper;
class SideNavbarHelper extends Helper {    
    public $helpers = ["Html"];
    public function defaultNavbar() {
        //some code here...
    }
}

I have confirmed that the plugin in loaded in bootstrap.php. 
Why am I getting the error? 

A:

You need to make sure there is an autoloader setup for your plugin classes.  Please refer this section in the documentation:
http://book.cakephp.org/3.0/en/plugins.html#autoloading-plugin-classes

