Logging in frontend giving 500 Internal Server Error in magento 1.9.2.1
<p>Whenever a customer is placing an order using credit card, then after that if the customer is trying to log-in or click on any of the top-link like 'checkout', 'account', then it is getting <code>500 Internal Server Error</code>.
It is majorly happening for pages with https</p>

<p>My PHP error log is filing this: </p>

<blockquote>
  <p>[26-Feb-2016 09:38:46 UTC] PHP Fatal error:  Call to a member function
  setFinalPrice() on a non-object in
  /www.example.com/app/code/core/Mage/Sales/Model/Quote/Item/Abstract.php
  on line 89</p>
</blockquote>

<p>Can anyone please suggest something?
It is very important for me to make it working otherwise live orders are blocked for customers</p>