Did you manage to resolve this? I don't see the error on the frontend any more.
While the error is wen $this->_item is tried to be used, the error would have actually been because of the previous line: https://github.com/OpenMage/magento-mirror/blob/magento-1.9/app/code/core/Mage/Sales/Model/Quote.php...
What would have needed investigating is what they standard collection call was returning a boolean (probably false). That's highly unusual. Do you have any extensions that are rewriting the quote model?
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