Magento扩展错误：调用非对象上的成员函数

I have a Magento extension which is supposed to add a donation to PayPal order, but it's throwing an error in Model/Observer.php. It is when people are done with PayPal and are redirected back to my website. The URL when this error is shown is /paypal/express/placeOrder/. The error is Fatal error: Call to a member function getBaseDonation() on a non-object in [path]/Model/Observer.php on line 215. Line 215 is inside the if (!$donation) {

public function addPaypalItem($observer)
    {
        $cart = $observer->getEvent()->getPaypalCart();
        $quote = $cart->getSalesEntity();

        $donation = $quote->getBaseDonation();
        if (!$donation) {
            $donation = $quote->getShippingAddress()->getBaseDonation() ? $quote->getShippingAddress()->getBaseDonation() : $quote->getBillingAddress()->getBaseDonation();
        }

        if ($donation > 0) {
            $cart->addItem(
                Mage::helper('donations')->__('Donation'),
                1,
                $donation
            );
        }

        return $this;
    }

How can I solve this non-object problem? Thank you!

First I think you should recheck the getShippingAddress() function to ensure it returns an object in our working flow.

If it is, let check if your extension already override to ShippingAddress object to input the BaseDonation value.

$quote->getBaseDonation() is different from $quote->getShippingAddress()->getBaseDonation();