Flutter StreamBuilder: correct errors

Need to correct errors in my app: Implemented StreamBuilder to display items in shopping cart's updated prices. Everything works as expected when one or two items are in cart, but when add third item I get the third item correctly but first and second items in cart display following error: "Bad state: field does not exist within the DocumentSnapshotPlatform. The relevant error-causing widget was StreamBuilder<DocumentSnapshot<Map<String, dynamic>>>". I believe that is a timing error (async / await), or some sort of pause needed. This is my code screenshot: