I am getting an error message on my laravel app: creating default object from empty value

I want to save some values in my transaction table. I stored the value of the pharmacy in a session and now am trying to save it. This is my controller;

Public function redirectToGateway(Request $request)
{
$old cart= Session:: get('cart');
$cart=new Cart($oldCart);
session ()->put ('user_id,$request->get('user_id));
session ()->put ('pharmacy',$request->get('pharmacy'));
$id= Session:: get('user_id');
$pharmacy= Session:: get('pharmacy');
$transhistory=Transaction:: find($id);
$transhistory->pharmacy=pharmacy;
$transhistory->email=$request->get('email');
$transhistory->amount=$request->get('amount');
$transhistory->cart=serialize($cart);
$transhistory->save();

via Chebli Mohamed