Warning: get_class() expects parameter 1 to be object, boolean given - magento

Question

I'm having a problem that's been in some way posted here before but never for this case. Usually this error occurs in App.php but for me it's occurring in /httpdocs/app/design/frontend/default/theme/template/catalog/category/view.phtml on line 42 which I've indicated below.
Does anyone have any idea on the fix for this? I'm a bit at a loss.
<?php
/**
* Category view template
*
* #see Mage_Catalog_Block_Category_View
*/
?>
<?php
$_helper = $this->helper('catalog/output');
$_category = $this->getCurrentCategory();
$_imgHtml = '';
$_category->setCanShowBlock(false);
$_category->setHasHeaderImage(false);
//////Line 42/////// if (get_class($this->getLayout()->getBlock('catalog.leftnav')) != 'Mage_Catalog_Block_Navigation') {
$_category->setCanShowBlock($this->getLayout()->getBlock('mana.catalog.leftnav')->canShowBlock());
}
if ($_imgUrl = $_category->getImageUrl()) {
$_imgHtml = '<img src="'.$_imgUrl.'" alt="'.$this->htmlEscape($_category->getName()).'" title="'.$this->htmlEscape($_category->getName()).'" />';
$_imgHtml = $_helper->categoryAttribute($_category, $_imgHtml, 'image');
$_category->setHasHeaderImage(true);
}
?>
<?php /*
<!-- <div class="page-title category-title">
<?php if($this->IsRssCatalogEnable() && $this->IsTopCategory()): ?>
<?php echo $this->__('Subscribe to RSS Feed') ?>
<?php endif; ?>
<h1><img src="<?php echo $this->getSkinUrl('images/' . $_helper->categoryAttribute($_category, $_category->getName(), 'name') . '.jpg') ?>" alt="<?php echo $_helper->categoryAttribute($_category, $_category->getName(), 'name') ?>" /></h1>
</div> -->
*/
Mage::getModel('core/session')->setHasAdditionalDescription(false);
?>
<?php echo $this->getMessagesBlock()->getGroupedHtml() ?>
<div id="category_top_image"<?php if ($_category->getCanShowBlock()): ?> style="margin-left: -280px;"<?php endif ?>>
<?php if($_imgUrl): ?>
<?php echo $_imgHtml ?>
<?php endif; ?>
<?php if(strtolower($_category->getHideTitle()) != 'yes'): ?>
<h1><?php echo $_helper->categoryAttribute($_category, $_category->getName(), 'name') ?></h1>
<?php endif ?>
<?php if($_description=$this->getCurrentCategory()->getDescription()): ?>
<div class="category-description std">
<?php echo $_helper->categoryAttribute($_category, $_description, 'description') ?>
</div>
<?php endif; ?>
</div>
<?php if($this->isContentMode()): ?>
<?php echo $this->getCmsBlockHtml() ?>
<?php elseif($this->isMixedMode()): ?>
<?php echo $this->getCmsBlockHtml() ?>
<?php if($_description=$this->getCurrentCategory()->getAdditionalDescription()): ?>
<?php Mage::getModel('core/session')->setHasAdditionalDescription(true) ?>
<div class="category-additional-description std">
<?php echo $_helper->categoryAttribute($_category, $_description, 'additional_description') ?>
</div>
<?php endif; ?>
<?php echo $this->getProductListHtml() ?>
<?php else: ?>
<?php if($_description=$this->getCurrentCategory()->getAdditionalDescription()): ?>
<?php Mage::getModel('core/session')->setHasAdditionalDescription(true) ?>
<div class="category-additional-description std">
<?php echo $_helper->categoryAttribute($_category, $_description, 'additional_description') ?>
</div>
<?php endif; ?>
<?php echo $this->getProductListHtml() ?>
<?php endif; ?>

Answer 1

Well, that error means that there is no block named catalog.leftnav on the page, so you should check your layouts. And actually this way of checking class is bad since it leads to such errors. Also there will be a problem, if some extension will rewrite Mage_Catalog_Block_Navigation class.
So I'm not sure why do you want to check the class of that block, but if you really need it, I would suggest something less risky and rewrite-friendly:
if ($block = $this->getLayout()->getBlock('catalog.leftnav') && $block instanceof Mage_Catalog_Block_Navigation) {
...
}