Magento2 Controllers – How to Fix Custom Controller Errors

I created a module shown in this post. But my own controller doesn't work. I've got this error message:

Recoverable Error: Argument 1 passed to
MP\MyModule\Controller\Index\CheckUserName::__construct() must be an
instance of Magento\Framework\App\Action\Context, instance of
Magento\Framework\ObjectManager\ObjectManager given, called in
D:\xampp\htdocs\magento2\vendor\magento\framework\ObjectManager\Factory\AbstractFactory.php
on line 97 and defined in
D:\xampp\htdocs\magento2\app\code\mp\MyModule\Controller\Index\CheckUserName.php
on line 35

Here is my controller code:

<?php
namespace MP\MyModule\Controller\Index;

class CheckUserName extends \Magento\Framework\App\Action\Action
{
    protected $_logger;
    protected $_objectManager;
    protected $_request;

/*
            \Psr\Log\LoggerInterface $logger, //log injection
            \Magento\Framework\App\Request\Http $request

        $this->_logger = $logger;

        $this->_logger->debug('CheckUserName_Constructor_Begin');

        $this->_request = $request;

        $this->_logger->debug('CheckUserName_Constructor_End');     


        */  

        /**
     * @var \Magento\Framework\View\Result\PageFactory
     */
    protected $resultPageFactory;

    /**
     * @param \Magento\Framework\App\Action\Context $context
     * @param \Magento\Framework\View\Result\PageFactory resultPageFactory
     */
    public function __construct(
        \Magento\Framework\App\Action\Context $context,
        \Magento\Framework\View\Result\PageFactory $resultPageFactory
    )
    {
        parent::__construct($context);
        $this->resultPageFactory = $resultPageFactory;
    }

    public function execute()
    {
        echo "Hello from Checkusername";
    }    
}
 ?>

Here're the error messages after deleting the var/generation folder:

Warning: ltrim() expects parameter 1 to be string, object given in D:\xampp\htdocs\magento2\vendor\magento\framework\Code\Generator\EntityAbstract.php on line 152

Notice: Uninitialized string offset: 0 in D:\xampp\htdocs\magento2\vendor\magento\framework\Autoload\ClassLoaderWrapper.php on line 81

Notice: Uninitialized string offset: 0 in D:\xampp\htdocs\magento2\vendor\composer\ClassLoader.php on line 317

Notice: Uninitialized string offset: 0 in D:\xampp\htdocs\magento2\vendor\composer\ClassLoader.php on line 349 exception 'Magento\Framework\Exception\LocalizedException' with message 'Source class "" for "Magento\Framework\App\Response\Http\Interceptor" generation does not exist.' in D:\xampp\htdocs\magento2\vendor\magento\framework\Code\Generator.php:171 Stack trace: #0 D:\xampp\htdocs\magento2\vendor\magento\framework\Code\Generator.php(100): Magento\Framework\Code\Generator->tryToLoadSourceClass('Magento\\Framewo...', Object(Magento\Framework\Interception\Code\Generator\Interceptor))
#1 D:\xampp\htdocs\magento2\vendor\magento\framework\Code\Generator\Autoloader.php(35): Magento\Framework\Code\Generator->generateClass('Magento\\Framewo...')
#2 [internal function]: Magento\Framework\Code\Generator\Autoloader->load('Magento\\Framewo...')
#3 D:\xampp\htdocs\magento2\vendor\magento\framework\ObjectManager\Factory\AbstractFactory.php(105): spl_autoload_call('Magento\\Framewo...')
#4 D:\xampp\htdocs\magento2\vendor\magento\framework\ObjectManager\Factory\Compiled.php(88): Magento\Framework\ObjectManager\Factory\AbstractFactory->createObject('Magento\\Framewo...', Array)
#5 D:\xampp\htdocs\magento2\vendor\magento\framework\ObjectManager\Factory\Compiled.php(130): Magento\Framework\ObjectManager\Factory\Compiled->create('Magento\\Framewo...')
#6 D:\xampp\htdocs\magento2\vendor\magento\framework\ObjectManager\Factory\Compiled.php(67): Magento\Framework\ObjectManager\Factory\Compiled->get('Magento\\Framewo...')
#7 D:\xampp\htdocs\magento2\vendor\magento\framework\ObjectManager\ObjectManager.php(57): Magento\Framework\ObjectManager\Factory\Compiled->create('Magento\\Framewo...', Array)
#8 D:\xampp\htdocs\magento2\vendor\magento\framework\App\Bootstrap.php(233): Magento\Framework\ObjectManager\ObjectManager->create('Magento\\Framewo...', Array)
#9 D:\xampp\htdocs\magento2\index.php(38): Magento\Framework\App\Bootstrap->createApplication('Magento\\Framewo...')
#10 {main}