Fatal error: Call to a member function toCurrency() on null in app\code\core\Mage\Directory\Model\Currency.php on line 270

[TivoSoho]

This has been troubling us for a while and hard to debug as after clearing cookies it is fine again. But I believe it comes from Aoe_BackendDefaultLanguage.

This is a sample from Magento's own core code:
Mage::getSingleton('adminhtml/session')->setLocale(Mage::app()->getLocale()->getLocaleCode());
And this is what you are doing in Aoe_BackendDefaultLanguage_Model_Observer:
$localeCode = Mage::getModel('core/locale')->getLocale();

And since in Mage_Adminhtml_Model_Observer we have so:

    public function bindLocale($observer)
    {
        if ($locale=$observer->getEvent()->getLocale()) {
            if ($choosedLocale = Mage::getSingleton('adminhtml/session')->getLocale()) {
                $locale->setLocaleCode($choosedLocale);
            }
        }
        return $this;
    }

How this manifests in the Mage_Directory_Model_Currency is that is calls for the currency($currency) function in Mage_Core_Model_Locale and that has:
self::$_currencyCache[$this->getLocaleCode()][$currency])
As getLocaleCode() returns an object, it will fail to set the values in the array, returns null and you cannot call methods on null - thus the error.

So just replace the getLocale() with getLocaleCode() and it should be fine.

[tomekjordan]

I got the same issue. product grid show the fatal in price column after I log in. is this resolved finally in official repo?

[WRedMedia]

same issue