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I wrote a module, it work if  I use return 'Hello World';  but does not work if I use return $this->display(__FILE__, 'mymodule.tpl');

Error: 

What is the problem?

Thanx for help!

The code:

<?php
if (!defined('_PS_VERSION_'))
    exit;

class MyModule extends Module
{
    public function __construct()
    {
        $this->name = 'mymodule'; /* This is the 'technic' name, this should equal to filename (mycustommodule.php) and the folder name */
        $this->tab = 'front_office_features'; /* administration, front_office_features, etc */
        $this->version = '1.0.0'; /* Your module version */
        $this->author = 'Tibor Darvalics'; /* I guess it was clear */
        $this->need_instance = 0; /* If your module need an instance without installation */
        $this->ps_versions_compliancy = array('min' => '1.6', 'max' => _PS_VERSION_); /* Your compatibility with prestashop(s) version */
        $this->bootstrap = true; /* Since 1.6 the backoffice implements the twitter bootstrap */

        $this->displayName = $this->l('My module'); /* This is the name that merchant see */
        $this->description = $this->l('This is my first module.'); /* A short description of functionality of this module */

        //$this->context->controller->addCSS($this->_path.'assets/css/mymodule.css', 'all');
  }

     public function initContent()
     {
         $this->context->smarty->assign('message', 'hello'); 
         $this->context->controller->addCSS($this->_path.'assets/css/mymodule.css', 'all'); 
        	parent::initContent();
 }

  public function initContent()
     {
         $this->context->smarty->assign('message', 'hello'); 
         $this->context->controller->addCSS($this->_path.'assets/css/mymodule.css', 'all'); 
       	parent::initContent();
   }
	
	public function install()
	{
	if (Shop::isFeatureActive()) {
        Shop::setContext(Shop::CONTEXT_ALL);
    }

    return parent::install() &&
        $this->registerHook('leftColumn') &&
        Configuration::updateValue('MYMODULE_NAME', 'mymodule');
	}
	
	public function uninstall() {
	return parent::uninstall();
	}
		
	public function hookDisplayLeftColumn($params)
	{
 	//return 'Hello World';
    
return $this->display(__FILE__, 'mymodule.tpl');
}
}
?>

mymodule.tpl

<!-- Block mymodule -->
<div id="mymodule_block_home" class="block">
  <h4>Welcome!</h4>
  <div class="block_content">
    <p>Hello,
           {if isset($message) && $message}
               {$message}
           {else}
               Nincs üzenet!
           {/if}
           !
    </p>
   
  </div>
</div>
<!-- /Block mymodule -->