title: getting "Fatal Error" when any function is call by $cart in Prestashop 1.7
body: <p>I'm facing an issue, where I'm getting 'Fatal error' after adding product.</p>
  <p><img src="https://i.stack.imgur.com/IdvZo.png" alt="enter image description here" /></p>
  <p><img src="https://i.stack.imgur.com/IOGkL.png" alt="enter image description here" /></p>
  <p>When I call any cart method, it leads to fatal error.</p>
  <p>PS: 1.7.8.2
  <a href="https://beta.claap.io/sendinblue/fatal-error-c-3YfMIiN6mG-AL7OcMGtoJMg" rel="nofollow noreferrer">https://beta.claap.io/sendinblue/fatal-error-c-3YfMIiN6mG-AL7OcMGtoJMg</a></p>