<field name="template-menu" type="hidden" default='1' />

 protected $defaults = array(
        'template-menu' => '0',
        'enable_current_id' => 0,
        // ...

public function renderMenu(&$menu)
{
    ob_start();

    if (isset($this->args['template-menu']) && $this->args['template-menu'] == '1'): ?>
        <div class="gf-menu-device-container responsive-type-<?php echo $this->args['responsive-menu']; ?>"></div>
    <?php endif; ?>
    <ul class="gf-menu <?php echo $this->args['class_sfx']; ?> l1"
      <?php if (array_key_exists('tag_id', $this->args)): ?>id="<?php echo $this->args['tag_id']; ?>"<?php endif; ?>>
      <?php foreach ($menu->getChildren() as $item) : ?>
    	  <?php $this->renderItem($item, $menu); ?>
      <?php endforeach; ?>
    </ul>
<?php
    return ob_get_clean();
}

public function renderMenu(&$menu)
{
    ob_start();
    $menuname = (isset($this->args['template-menu']) && $this->args['template-menu'] == '1') ? 'gf-menu gf-splitmenu' : 'menu';
    ?>

    <?php // this if is probably wrong -> further tests required
      if ($menu->getChildren()) : ?>
        <?php if (isset($this->args['template-menu']) && $this->args['template-menu'] == '1'): ?>
            <div class="gf-menu-device-container responsive-type-<?php echo $this->args['responsive-menu']; ?>"></div>
        <?php endif;
           // class_sfx is the class suffix, so it must be appended to the original class ?>
        <ul class="<?php echo $menuname; ?><?php echo $this->args['class_sfx']; ?> l1"
          <?php if (array_key_exists('tag_id', $this->args)): ?>id="<?php echo $this->args['tag_id']; ?>"<?php endif; ?>>
          <?php foreach ($menu->getChildren() as $item) : ?>
        	  <?php $this->renderItem($item, $menu); ?>
          <?php endforeach; ?>
        </ul>
    <?php endif; 
    
    return ob_get_clean();
}

public function renderMenu(&$menu)
{
    ob_start();
    
    $menuname =  'menu';
    $sfxClass = isset($this->args['submenu-class_sfx']) ? $this->args['submenu-class_sfx'] : '';
    
    
    if( $menu->getChildren() ) : // do not output anything if menu has no elements
    
        // only if template menu is active, activate the top main layout and the responsive version
        if( isset($this->args['template-menu']) && $this->args['template-menu'] == '1' &&
          isset($this->args['style']) && $this->args['style'] == 'mainmenu'){
        
            $menuname =  'gf-menu gf-splitmenu';
            $sfxClass = isset($this->args['mainmenu-class_sfx']) ? 
                $this->args['mainmenu-class_sfx'] : '';
            ?><div class="gf-menu-device-container responsive-type-<?php 
                echo $this->args['responsive-menu'];?>"></div>
            <?php
        }
        
        // class_sfx is the class suffix, so it must be appended to the original class ?>
        <ul class="<?php echo $menuname; ?><?php echo $sfxClass; ?> l1"
          <?php if (array_key_exists('tag_id', $this->args)): 
            ?>id="<?php echo $this->args['tag_id']; ?>"       <?php endif; ?>>
        <?php foreach ($menu->getChildren() as $item) : ?>
          <?php $this->renderItem($item, $menu); ?>
        <?php endforeach; ?>
        </ul>
        <?php endif; // ( $menu->getChildren() )
    
    return ob_get_clean();
}