<?php
/**
 * ChannelEngine Payment Method
 */
class Tritac_ChannelEngine_Model_Payment_Method_Channelengine extends Mage_Payment_Model_Method_Abstract {

    /**
     * System payment method code
     *
     * @var string
     */
    protected $_code = 'channelengine';

    public function isAvailable($quote = null)
    {
        if(!is_null($quote) && $quote->getIsSystem() && $quote->getPayment()->getMethod() == $this->_code) {
            return true;
        }

        return false;
    }
}

<?php
/**
 * ChannelEngine Payment Method
 */
class Tritac_ChannelEngine_Model_Payment_Method_Channelengine extends Mage_Payment_Model_Method_Abstract {

    /**
     * System payment method code
     *
     * @var string
     */
    protected $_code = 'channelengine';

    public function isAvailable($quote = null)
    {
        if($quote->getIsSystem() && $quote->getPayment()->getMethod() == $this->_code) {
            return true;
        }

        return false;
    }
}
--- 
+++ 
@@ -13,7 +13,7 @@
 
     public function isAvailable($quote = null)
     {
-        if($quote->getIsSystem() && $quote->getPayment()->getMethod() == $this->_code) {
+        if(!is_null($quote) && $quote->getIsSystem() && $quote->getPayment()->getMethod() == $this->_code) {
             return true;
         }
 
Fix possible null error when isAvailable is called without a quote